Explanation:

✅ D. PhoneExists
This is the only supported field type for filtering synchronized data extensions in the Synchronized Data Sources configuration. It’s a Boolean field, which can be used directly in the synchronization setup UI to filter data during the sync process. Marketing Cloud only allows fields of data type Boolean, Date, or String to be used as filters in Synchronized Data Extensions. Since PhoneExists is already a Boolean and its logic is validated in Sales Cloud, it is reliable and directly usable during sync setup.

❌ A. ValidPhone
Although ValidPhone is also a Boolean, it’s a formula field, not a raw Boolean field. Marketing Cloud does not allow Salesforce formula fields to be used as filters in synchronized data extensions. This restriction exists because formula values can change based on logic, which may not behave consistently during synchronization. Even if the formula evaluates to a Boolean value, it’s still ineligible for use in filtering synchronized DEs. Therefore, this field can’t be used to limit what records are pulled into Marketing Cloud.

❌ B. Phone
Phone is a text field with a specialized data type (Phone), and while it contains the actual phone number, it’s not supported in sync filtering because it’s not a Boolean, Date, or String data type that meets the Marketing Cloud synchronization filter criteria. Even if it’s not blank, the sync configuration does not support checking for empty/non-empty text fields directly. Hence, relying on the presence of a value in the Phone field is insufficient during configuration.

❌ C. ContactType
This is a text field that contains values like "Phone", but it’s not directly tied to the presence of an actual phone number. For example, someone could have a ContactType of "Phone" even if their phone field is blank. Also, filtering by ContactType may exclude valid leads or include false positives. Although it's technically a String and could be used in some filters, it’s not the most accurate or recommended choice for filtering based on phone number presence.