Comprehensive and Detailed Explanation:
The correct answer is B - Use an automation with Tracking Extract.
* Tracking Extract Activity in Automation Studio can pull detailed send status data, including 'Not Sent' reasons (bounces, held status, validation errors).
* From the extracted tracking file, you can create reports or data extensions specifically identifying Not Sent contacts.
#Why others are wrong:
* A: No built-in standard report in Salesforce Marketing Cloud specifically called "Subscribers Not Sent to."
* C: Discover reports focus on aggregate metrics, not granular send statuses.
* D: _Sent Data View records only successful send attempts; 'Not Sent' contacts are not logged in _Sent.
Exact Extract from Salesforce Official Documentation:
Salesforce Help - Tracking Extract Activity Overview:
"Use the Tracking Extract Activity to extract detailed subscriber-level data including message not sent events, undelivered messages, and reasons for failure." (Source:https://help.salesforce.com/s/articleView?id=sf.mc_as_tracking_extract_activity.htm)