Answer:
c. Contacting me by December 1 about your support of this new benefits plan will allow me to present this proposal at the next strategic planning session.
Step-by-step explanation:
The closing for a request should indicate a deadline for receiver, which can help to engage receiver in sender’s process.
Even though in this scenario, the sender is requesting for a support which might be not receiver’s responsibility to accept the benefit plan, but the sender can smartly appreciate the receiver if he can have the support by December 1.
I suppose with this closing in an email, receiver will put this mail in his working calendar or prioritized list if he really care and want to support this plan.