Auto Suppression List is the correct answer. An auto suppression list is a list of subscribers who should not receive certain types of emails, such as promotional or transactional emails. Auto suppression lists can be applied at the sender profile level or at the send classification level. If a subscriber is on an auto suppression list that is applied to a triggered send, they will not receive the email, even if they are in the target data extension. The _subscribers data view will show their status as 'Held' for that sender profile or send classification. You can learn more about this in the Manage Subscribers module on Trailhead.