This problem can be figured out using the idea of DOTS:  Difference of Two Squares has a + and a -, such as x^2 – 9 = (x + 3) (x – 3).  Therefore only answers 3 or 4 are possible!  Answers (1) and (2) will result in a trinomial with 3 terms but we only want 2 terms.

Looking at answer (4), (8x^2)(8x^2) = 64x^4 and we are looking for 16x^4.