Answer:
D. difference of squares
Step-by-step explanation:
Both of the terms x^2 and 9 are squares, so their difference is the difference of squares. The factorization of that is a "special form" that can be usefully memorized.
a^2 -b^2 = (a -b)(a +b)
x^2 -9 = (x -3)(x +3)
