Comprehensive and Detailed Explanation:
The correct answer is D - A GUID or another generated ID for Contact Key.
* Best practice is to use a channel-agnostic, stable, unique identifier for Contact Key.
* Email addresses and phone numbers can change. A GUID (Globally Unique Identifier) or system- generated ID ensures persistent identification across email, SMS, push, and other channels.
#Why others are wrong:
* A: Physical Address isn't unique and can change.
* B: Phone number can change and is not unique to a single identity.
* C: Email addresses are prone to change over time (job switch, preference change).
Exact Extract from Salesforce Official Documentation:
Salesforce Help - Subscriber Key and Contact Key Best Practices:
"Use a non-channel specific, unique, and persistent identifier, such as a generated GUID or system ID, as the Subscriber Key (Contact Key) across all messaging channels." (Source:https://help.salesforce.com/s/articleView?id=sf.mc_es_contact_key_best_practices.htm)
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