The marketer should mark the field as the Primary Key to ensure the Email Address field values are unique1. A primary key is a field that uniquely identifies each record in a data extension2. By marking the Email Address field as the primary key, the marketer can prevent duplicate email addresses from being imported or added to the data extension3. This can help the marketer maintain data quality and avoid sending multiple emails to the same subscriber.
Using Email Address as Subscriber Key is not the correct answer, as it does not ensure the Email Address field values are unique. A subscriber key is a field that uniquely identifies each subscriber in the Marketing Cloud account4. By using Email Address as Subscriber Key, the marketer can link the subscriber's email address to their profile and preference attributes, but they cannot prevent duplicate email addresses from being stored in different data extensions5.
Selecting 'Deduplicate by Email' checkbox is also not the correct answer, as it does not ensure the Email Address field values are unique. This option is available when importing data into a data extension from a file or another data extension. By selecting this option, the marketer can avoid importing duplicate email addresses based on an existing email address field in the destination data extension. However, this option does not affect the structure or properties of the Email Address field itself. References := 1: Data Extension Field Properties - Salesforce 2: Data Extensions - Salesforce 3: Importing Data into Data Extensions - Salesforce 4: Subscriber Key - Salesforce 5: Subscriber Key Migration Guide - Salesforce : Deduplicate Subscribers by Email Address - Salesforce