Acommunications objective is:

a. the audience the ad is targeting.

b. what people say after they see an ad.

c. the essence of an ad's message.

d. what people buy after viewing an ad.

A communications objective is basically an advertisement or program that is creating awareness, imparting knowledge, projecting images, or affecting sales. 

I'm crossing out B and D because there aren't necessary the answer this question is looking. A and C is a highly possible answer for both choices because it is what a communications objective basically is. 

Although I may be wrong in this, but I'm aiming towards answer C, "The essence of an ad's message."