Summary:
With three sections and four possible content blocks per section, there are 4³ = 64 possible email combinations. When sending to over one million contacts, the only scalable and reliable way to guarantee no subscriber receives a completely blank email is to ensure every dynamic section has a default/fallback content block that renders when no personalized content is matched. Relying solely on manual testing or preview cannot cover all edge cases at this scale.

Correct Option:

D. Confirm every version has default content
This is the only true best practice for large-scale dynamic emails. Every dynamic section must include a fallback (default) content block using techniques like ContentBlockByKey/ID with a default key, ClaimRow, or IF/ELSE logic with a final ELSE that outputs generic content. This guarantees that even if all personalization rules fail, the section will never be blank, preventing an empty email.

Incorrect Option:

A. Send a test of every possible version using Test Send
Impossible at scale. Manually testing 64 versions via Test Send is impractical and still misses real-world data mismatches.

B. Review every possible version using Subscriber Preview
Subscriber Preview can check many combinations but cannot realistically verify all 64 versions, especially when data varies across one million subscribersons. It also does not catch runtime failures.

C. Create separate emails for each version
Creating 64 separate emails defeats the purpose of dynamic content, dramatically increases maintenance overhead, and is not a viable solution.

Reference:
Salesforce Help – Dynamic Content Best Practices

Explanation:

✅ D. SELECT SubscriberKey FROM _Open WHERE JobID = '1234'
This is the correct query when building an audience based on email opens. The _Open data view stores records of all email open events, and the SubscriberKey column is the unique identifier for a contact in Contact Builder and across the account. Using SubscriberKey ensures that the resulting data set is compatible with other contact-centric operations, such as filtering data extensions or building audiences in Journey Builder. Including a WHERE JobID = '1234' clause ensures you're narrowing the query to opens for a specific send/job.

❌ A. SELECT * FROM _Open WHERE ListID = '1234'
This query is problematic for two reasons. First, using * is inefficient and pulls all columns, which may not be necessary. Second, ListID is only applicable in list-based sending (which is less common in enterprise setups that use data extensions). Also, targeting by ListID does not guarantee accuracy, especially in DE-centric environments. Since the question involves identifying opens from a specific email, filtering by JobID is more precise.

❌ B. SELECT * FROM _Open WHERE JobID = "1234"
Although this query includes the correct filtering clause (JobID = "1234"), it still has two issues: it uses double quotes instead of single quotes (which is invalid in most SQL dialects used in SFMC), and it selects *, which is not optimal. Pulling all columns is unnecessary when you only need a specific identifier like SubscriberKey to build an audience. It’s better practice to retrieve only what's needed for performance and clarity.

❌ C. SELECT SubscriberID FROM _Open WHERE JobID = "1234"
This query retrieves the SubscriberID, which is an internal system-generated value and not always consistent across accounts, especially in environments using Contact Builder with SubscriberKey-based relationships. Modern Marketing Cloud development practices recommend using SubscriberKey over SubscriberID for better portability and account consistency. Additionally, this query also uses double quotes, which can cause syntax errors depending on the environment.

Explanation:

When using the /contacts/ route of the REST API to update records in a data extension, it's important to ensure that the data extension is properly linked to the Contact model in Contact Builder. This is done by linking the data extension to an Attribute Group in Contact Builder.

Attribute Groups in Contact Builder are used to group together related data extensions and associate them with contacts, which ensures that they are available for use in the REST API for contact management.

Once a data extension is linked to an Attribute Group, you can use the /contacts/ route to update the associated records in that data extension. This is because the API can then reference the attributes (fields) in the data extension that are part of the contact model.

Why Not the Other Options?

A. Each contact should already exist in All Subscribers:
While it's important for contacts to exist, the REST API allows you to create or update contacts independently of whether they are in All Subscribers. All Subscribers is used for email-sending purposes, but it’s not a prerequisite for using the /contacts/ route in the API.

B. Journey Builder should be configured to use the data extension:
This is not required for making API calls to update data extensions. Journey Builder is used to orchestrate customer journeys, but you don't need it to update records in a data extension via the REST API.

C. Contact Key should be equal to Subscriber Key in the underlying data extensions to ensure proper joining:
While it is good practice to keep the Contact Key equal to the Subscriber Key for consistency and ease of use, the Contact Key in the Contact model does not necessarily need to match the Subscriber Key in every data extension. The Contact Key is used for identification in the Contact model, and as long as the data extension is properly linked in Contact Builder, you can update it.

Best Practice:
Link the data extension to a Contact model in Contact Builder through an Attribute Group. This will allow you to properly update and manage contact data in the data extension using the REST API.

Explanation:

The Contact Delete process in Salesforce Marketing Cloud is used to remove contacts from the system, but it does not delete rows from data extensions. It only removes the contact from the system (including its subscriber record, preferences, etc.) within the All Contacts database.

Option A (Sendable data extension with SubscriberKey and EmailAddress fields): The Contact Delete process operates at the contact level, not at the data extension level. It does not affect the data extension rows.

Option B (Non-sendable data extension with SubscriberKey field): If the data extension is non-sendable, it is not linked to a subscriber or contact, and thus would not be impacted by the Contact Delete process anyway.

Option D (Sendable data extension with SubscriberKey field): Even for a sendable data extension, the Contact Delete process only removes the contact record. It does not remove the associated rows from the data extension unless you explicitly run an additional query or process to delete them.

Additional Context:

To delete rows from a sendable or non-sendable data extension, a developer can either use:

1. SQL queries to remove rows from data extensions.
2. API calls to delete subscriber records and their associated data.

Explanation:

B. %%SendBoo1 < 1:

If the field SendBoo1 is a Boolean field (which means it contains True or False values, or 1 and 0), comparing it directly with 1 can determine whether the condition is true or false. For example, if the value is 0 (False), the exclusion condition will be met, and the email won't be sent.

Example:
%%SendBoo1 < 1

This will evaluate to true if SendBoo1 is false or 0 and exclude the contact from receiving the email.

C. %%=Lookup('Excluded', 'SendBoo1'.SubscriberKey, _SubscriberKey) =%%:

This method uses the Lookup function to compare the Boolean field SendBoo1 from the Sendable Data Extension and check if it matches a specific condition.
This query looks up the value of SendBoo1 for a given SubscriberKey and compares it against a certain condition to determine whether or not to exclude the subscriber from receiving the email.

Example:
%%=Lookup('Excluded', 'SendBoo1'.SubscriberKey, _SubscriberKey)%%

Why Not the Other Options?

A. %%SendBoo1%%:

This expression would output the value of SendBoo1 but does not include any logic for comparison or exclusion. The exclusion logic should compare the value to a specific condition (like True, False, 1, or 0), not just display the value.

D. %%=Lookup ('Excluded’ , SendBoo1'. _Subscribekey'. _Subscribekey) =%%:

This syntax is incorrect because it has misplaced quotes and syntax issues (e.g., extra periods and improper usage of underscores). The correct syntax should be used for the Lookup function, and the comparison should involve matching field names correctly, as in option C.