答案：C

解析：

Choice C is correct. First follow the order of operations to get the following.

4-4a-(5+a)(5-a)=-21-4a+a^2

Since the answer choices do not match this, look for more factors.

The factors of -21 that add to -4 are -7 and 3.

Therefore, the answer is (a-7)(a+3).