Comprehensive and Detailed Explanation:

The correct answer is B — Contact ID.

When sending from Salesforce CRM using Marketing Cloud Connect and a Report, the Contact ID (or Lead ID if working with leads) must be included.

Marketing Cloud Connect uses the Contact ID as the unique identifier to associate the subscriber with the correct record in Salesforce.

Without the Contact ID, Marketing Cloud cannot perform the send correctly.

❌ Why others are wrong:

A: Individual ID is related to Salesforce's Individual Object for GDPR compliance, not standard send flows.

C: CampaignMember ID is used in Campaign context, not for newsletter signup or basic contact sends.

D: External ID is a custom field — not required or recognized by the Send Flow.

Exact Extract from Salesforce Official Documentation:

Salesforce Help — Marketing Cloud Connect Send from Salesforce:

"When sending using a Report from Salesforce CRM to Marketing Cloud, include the Contact ID or Lead ID in the report. Marketing Cloud uses this ID to map the subscriber and ensure accurate sending."

(Source: https://help.salesforce.com/s/articleView?id=sf.mc_co_send_from_salesforce_reports.htm )