Which feature would allow NTO to monitor whether all records were added to the target data structure each day?

Northern Trail Outfitters (NTO) keeps their subscribers in sync with their external database via the import of a CSV file which is dropped to the of Marketing Cloud SFTP each day. However, NTO has realized the number of subscribers being sent emails is considerably lower than the number they were expecting based on records in their database.

Which feature would allow NTO to monitor whether all records were added to the target data structure each day?
A . External Key within the Import File Activity
B . Run Completion within the File Drop Automation
C . Runtime Error within the File Drop Automation
D . Notation Settings within the Import File Activity

Answer: D