How do you make sure users have to confirm via e-mail that they indeed wish to subscribe?

You are setting up a new marketing campaign and want to make sure all subscriptions are genuine.

How do you make sure users have to confirm via e-mail that they indeed wish to subscribe?
A . Enable double opt-in in the subscription.
B. Enable e-mail verification in the subscription.
C. Enable the required flag in the subscription.
D. Enable the active flag in the subscription.

Answer: C