The feature that the app builder should use to allow users to create the opportunity is a quick action. A quick action is a type of custom button or link that can create or update records, log calls, send emails, or launch flows from a record page or a global menu. The app builder can create a quick action on the account object that creates an opportunity record with pre-populated fields. Option B is incorrect because a default action is not a valid term, but rather a standard action that appears on every record page by default, such as Edit or Delete. Option C is incorrect because a custom button is not suitable for this requirement, as custom buttons can only display a URL or execute JavaScript when clicked. Option D is incorrect because a custom link is not suitable for this requirement, as custom links can only display a URL or execute JavaScript when clicked.

The validation rule should check if the user role is not Marketing, and if the Retail Opt In field has been changed to Yes. The ISCHANGED function returns true if the field value has been changed, and the ISPICKVAL function returns true if the field value matches a specified picklist value.