When joining a data extension in Contact Builder, in which scenario should a consultant mark the Use as Root checkbox?

When joining a data extension in Contact Builder, in which scenario should a consultant mark the Use as Root checkbox?
A . Supplemental data for an interaction in Journey Builder.
B . One-to-Many Relationship to the Contact Record.
C . Data for both Email and MobileConnect sends.
D . Database of record other than All Subscribers.

Answer: D