Northern Trail Outfitters (NTO) plans on sending SMS and push notifications together with emails as...

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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