Explanation:

When deploying Process Builder flows or Flows through a Change Set, Salesforce does not provide built-in UI tools to directly show their test coverage like it does for Apex classes. However, ensuring that flows are covered by Apex tests is still important — and can be done using the Tooling API and SOQL.

🔹 Why Option D is Correct:

The Tooling API provides access to metadata and test coverage data not available through the standard UI.
Developers can run SOQL queries (e.g., against FlowCoverage or ApexTestResult objects) to determine which flows have coverage.
This method gives granular and programmatic access to validate that a flow is being tested by Apex code.
It is the only reliable method to track coverage of flows during deployments.

❌ Why the Other Options Are Incorrect:

A. Use the Flow properties page:
This page does not show test coverage information. It’s mainly used for versioning and metadata.

B. Use the Code Coverage Setup page:
This page shows Apex class coverage, not flow or process builder coverage.

C. Use the Apex TestResult class:
This class provides information about Apex test execution but not specifically about Flow coverage.

Explanation:

To provide test coverage for Apex code that makes callouts to an external system, a developer needs to mock the callout to prevent actual communication with the external service during the test.

The correct approach is:

B. Write a class that implements the HTTPCalloutMock interface.
The HTTPCalloutMock interface provides the framework for defining mock responses for HTTP callouts in Apex tests. By creating a class that implements this interface, you can specify the HttpResponse that your callout code should receive, allowing you to test the callout's logic without actually connecting to an external system.

Why other options are incorrect:

A. Write a class that extends HTTPCalloutMock. HTTPCalloutMock is an interface, not a class, so it must be implemented, not extended.
C. Write a class that implements the WebserviceMock interface. The WebserviceMock interface is used for mocking responses for Apex callouts to external SOAP web services, not for general HTTP callouts (REST or SOAP when using HttpRequest and HttpResponse).
D. Write a class that extends WebserviceMock. Similar to option A, WebserviceMock is an interface, so it must be implemented, not extended.

Explanation:

In Salesforce, each Apex trigger must have at least 1% direct test coverage to be deployed to production—even if its associated helper class has high coverage. The test methods must explicitly invoke or indirectly trigger the logic within the Apex trigger, not just the helper class alone.

To successfully deploy:

Write or update test methods to create/modify records in a way that causes the trigger to fire.
This will provide direct coverage of the trigger body.

❌ Why Not the Other Options?

B. Run the tests using the 'Run All Tests' method
🔴 Just running tests won’t help unless the trigger itself is exercised. If your tests don’t touch the trigger, coverage remains 0%.

C. Remove the failing test methods from the test class
🔴 Removing tests doesn’t improve coverage—it reduces it. This won’t fix the root cause and can lead to even more test failures.

D. Increase the test class coverage on the helper class
🔴 The helper class may have 95% coverage, but that doesn’t count toward the trigger. You must explicitly test the trigger logic.

Explanation:

Validation rules are enforced by the Salesforce platform at the database level. When a validation rule fails during a DML operation (like insert, update), Salesforce throws a system-defined DmlException. The standard behavior of Visualforce, when coupled with a custom controller, is to automatically handle these exceptions and display any validation rule errors (or other platform errors) through the component.

Let's break down why this is the correct answer and why the others are not:

Why B is Correct: The component is specifically designed to display all information, confirmation, and error messages that are generated by the Salesforce platform. This includes validation rule violations, required field missing errors, duplicate rule violations, and other DML exceptions. By simply including this tag on your Visualforce page, any error that occurs during a DML operation in the controller's method will be displayed automatically to the user without any need for additional code to catch and parse the exception.

Why A is Incorrect: While you could create a custom controller attribute (e.g., a String variable) and manually set an error message in a catch block, this is entirely unnecessary and error-prone. The platform already provides the error messages automatically. Manually doing this would require catching the exception, parsing the error message, and then redirecting it to the page, which is what does for you automatically.

Why C is Incorrect: Using a try/catch block is part of the process, but it is not the complete solution on its own. If you simply catch the DmlException, the error is caught by your code but is never displayed to the user unless you explicitly take the error messages from the exception and add them to the page. The component is the standard, built-in mechanism for displaying those errors. A custom exception class is not needed for standard validation rule errors.

Why D is Incorrect: The choice of DML method (Database.upsert(), insert, etc.) does not change how validation rule violations are handled. All DML operations will respect and enforce validation rules. While Database.upsert() allows for partial success processing in bulk operations, for a single record save operation on a Visualforce page, its behavior regarding error handling and display would be the same as a standard upsert statement. The key to displaying the error remains the presence of the component.

Reference:
Visualforce Component Reference: apex:pageMessages - This component displays all messages that were generated for all components on the current page.

The underlying mechanism is that when a DML exception occurs, the platform adds the error messages to the ApexPages message list. The component simply renders these messages.

Explanation:

The requirement states that "Orders are only shipped when all of the Line Items are available," meaning the estimated ship date for the Order must be the latest (most distant in the future) availability date among all its associated Line Items.

To achieve this declaratively in Salesforce, the best method is:

D. Use a Max Roll-Up Summary field on the Latest availability date fields.

Here's why:

Roll-Up Summary Fields: These fields are specifically designed to calculate values from related detail records and display the aggregate on the master record.
MAX aggregation type: When defining a Roll-Up Summary field, you can choose MAX as the aggregate type. If applied to a Date or Date/Time field on the detail object (Line Item), it will automatically find the latest date among all related detail records.
This will automatically calculate the latest Availability Date from all Line Item records and store it on the Order record as the Estimated Ship Date.

Let's look at why the other options are incorrect:

A. Use a CEILING formula on each of the Latest availability date fields. CEILING is a mathematical function for rounding numbers. It's not used for date comparisons or finding the maximum date across multiple records.
B. Use a DAYS formula on each of the availability date fields and a COUNT Roll-Up Summary field on the Order. DAYS calculates the difference between dates. COUNT counts records. Neither of these helps in identifying the latest date among multiple related records.
C. Use a LATEST formula on each of the latest availability date fields. There is no standard "LATEST" formula function in Salesforce that works across multiple child records to find the maximum date. Formula fields operate on a single record or its direct parent, not across all children.