How should the administrator require this field during the negotiation stage for sales users but allow their managers to make changes?

Once an opportunity reaches the negotiation stage at Cloud Kicks, the Amount field becomes required for sales users. Sales managers need to be able to move opportunities into this stage without knowing the amount.

How should the administrator require this field during the negotiation stage for sales users but allow their managers to make changes?
A . Assign the Administrator profile to the managers.
B . Make the field required for all users.
C . Configure a validation rule to meet the criteria.
D . Create a formula field to fill in the field for managers.

Answer: C