A customer wants to report on 'Not Sent' contacts in Journey Builder.

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 )