Comprehensive and Detailed Explanation:
The correct answers are B and D.
* B - CRM ID: A unique identifier across CRM and Marketing Cloud.
* D - Loyalty Program Number: Another strong candidate for subscriber key if it is unique across systems.
Important! Subscriber Key must be unique, non-changing, and consistent across channels.
#Why others are wrong:
* A: Email can change over time; using email address as Subscriber Key can cause serious identity issues.
* C: Mobile Device ID is specific to mobile channels, not a cross-channel identifier.
Exact Extract from Salesforce Official Documentation:
Salesforce Help - Subscriber Key Best Practices:
"The subscriber key should be a unique, unchanging identifier such as a CRM ID or Loyalty ID to maintain consistent cross-channel communication." (Source:https://help.salesforce.com/s/articleView?id=sf.mc_es_subscriber_key_best_practices.htm)
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