What is the answer when you factor the polynomial completely?

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.