Factor x^2 - 16
a. (x + 4) (x - 4)
b. cannot be factored
c. (x + 4)^2
d. (x - 4)^2

Your answer is A. (x + 4)(x - 4)
This expression is what people call a difference of two squares, basically meaning that because the end number is a square number, and there is no x term, that the brackets must include and positive and negative form of whatever the square root of that number is. In this case, the square number is 16, so in the brackets you need -4 and +4, along with the x's.

I hope this helps!