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