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.
