What should a consultant recommend as a unique identifier for each subscriber?

Northern Trail Outfitters (NTO) plans on sending SMS and push notifications together with emails as part of a new customer onboarding journey.

What should a consultant recommend as a unique identifier for each subscriber?
A . A common field like Physical Address for Contact Key.
B. A number field such as subscriber’s phone number for Contact Key.
C. An email address for Contact Key.
D. A GUID or another generated ID for Contact Key.

Answer: D

Explanation:

To send SMS and push notifications together with emails as part of a new customer onboarding journey, Northern Trail Outfitters should use a GUID or another generated ID for Contact Key. A Contact Key is a unique identifier for each contact in Marketing Cloud across different channels and platforms. A GUID or another generated ID is recommended as a Contact Key because it is persistent, consistent, and channel-agnostic.

References: https://help.salesforce.com/s/articleView?id=sf.mc_co_subscriber_key.htm&type=5