Explanation
To resolve the issue of not being able to see the audience file in the data extension when scheduling the send, the admin should confirm that the data extension is marked as Sendable. A sendable data extension is one that has an email address field and a subscriber key field that can be used for sending emails. If the data extension is not marked as sendable, it will not appear as an option when selecting an audience for an email send. The other options are incorrect because:
* The data extension contains a Salesforce ID. This is not a requirement for sending emails from a data extension. A Salesforce ID is only needed if NTO wants to use Marketing Cloud Connect to sync data between Marketing Cloud and Salesforce CRM.
* The Data extension is marked as Sendable and Testable. This is not a valid option for marking a data extension. A data extension can only be marked as sendable or non-sendable; there is no testable option.
* The data extension is linked using the Contact Key. This is not a requirement for sending emails from a data extension. The Contact Key is only needed if NTO wants to use Contact Builder to link data across different sources and channels.