Which classification corresponds to a child under 21 who is in the member's custody?

The classification that pertains to a child under 21 who is in the member's custody is B. This classification signifies that the child is dependent on the member for support and care, which aligns with military policies for dependents. Such designations are crucial as they determine eligibility for various benefits and services provided to the family members of military personnel.

In this context, "B" specifically indicates that the individual is a minor or dependent living with the service member, and this status is significant for assignments, benefits, healthcare access, and other military entitlements. Understanding this classification helps ensure that service members and their families receive appropriate support and resources while navigating military life.