Comprehensive and Detailed Explanation:
The correct answer is C - An external system to receive and confirm callback and subscriptions.
* Marketing Cloud's Transactional Messaging API supports Event Notification Service (ENS), where an external endpoint (callback URL) can subscribe to event notifications like "Not Sent."
* You need an external system capable of receiving real-time POST messages (JSON format) from Marketing Cloud and sending 2xx HTTP status responses to confirm receipt.
# Why others are wrong:
* A: There is no "Not Sent Notification activity" inside Transactional Journeys - it's API-based event notifications.
* B and D: Datorama and Interaction Studio are analytics and personalization tools, not real-time event handling systems.
Exact Extract from Salesforce Official Documentation:
Salesforce Help - Transactional Messaging Event Notification Service:
"An external system must be configured to subscribe to notifications and must be capable of responding to callbacks with an HTTP 2xx status. Event types include Sent, Delivered, Not Sent, etc." (Source: https://help.salesforce.com/s/articleView?id=sf.mc-transactional-api-event-notifications.htm)
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