Northern Trail Outfitters receives a nightly encrypted unsub file to their Marketing Cloud SFTP from...

Because the incoming file is encrypted, a File Transfer activity must run first to decrypt/decompress it from the Enhanced SFTP location into the Safehouse before it can be processed, which rules out options B and C, which begin directly with an Import activity. The decrypted file is then brought in via a Data Copy or Import File activity into a staging data extension containing the third-party platform ' s identifiers (email addresses, since that ' s what the external unsub file will contain). Because NTO does not use Email Address as Subscriber Key, a Query Activity is required next to join this staged file against existing subscriber records to correctly resolve the corresponding internal Subscriber Key for each matched email address — without this step, the unsubscribe action would have no reliable way to target the correct internal records. A Data Extract Activity then produces a properly formatted unsubscribe file keyed on Subscriber Key, a second File Transfer moves that file to the location the unsubscribe process expects, and a final Data Copy or Import File activity performs the actual unsubscribe action against the All Subscribers List using the correctly resolved Subscriber Key values. Option A skips the essential Query step needed to resolve identity when Subscriber Key differs from Email Address. Reference: File Transfer Activity, Query Activity for Key Resolution, All Subscribers Unsubscribe Import.