the answer is D. it cannot be factored because the 16 is positive.
if it was negative it could be solve like that
X^2 - 16=
(x+4)(x-4)
x^2 -4x+4x-16
then the -4x and the +4x would cancelled out and you would left with:
x^2-16.
since it is positive it cannot be factored.
~hope i helped.