What should the customer have in place to ensure this solution can be implemented?

A customer wants to set up a real-time, API-driven way to be alerted when transactional messages fail to send out of Marketing Cloud. They are currently in the process of hiring a full-time developer but want an implementation in place for the developer to maintain.

What should the customer have in place to ensure this solution can be implemented?
A . A Transactional Journey with the Not Sent Notification activity included
B. A Datorama instance to ingest their data
C. An external system to receive and confirm callback and subscriptions
D. An Interaction Studio instance to ingest their data

Answer: C