Which polynomial is prime?
x2-36
x2+16
x2-7x+12
x2-x-20

Prime means can't be factored

first one can be factored to (x-6)(x+6)

2nd one can be forced into (x-4i)(x+4i) (might not be acceptable because used imaginary numbers)

3rd can be (x-3)(x-4)

4th is (x-5)(x+4)

2nd one is answer because you can't do it unless you use imaginary numbers