Hey there Jack!
So, in the question above, we are told to find to sentence that actually uses the prefix (a-) in the right correct manner. So, also, one thing to take to mind is that the prefix (a-) is taught to be know as (unknown) or also (not subjected).
By looking at your option's above, your correct answer would (The author wished to remain anonymous and did not sign his letter.)
The reason why (c) would be your correct answer is because the word (anonymous) is cleared to be known as (unsubjected) or unknown. The question was stating to find a word in the following sentence that would make the prefix (a-) actually mean something.This is why (c) is your answer.
Hope this helps you!