Which polynomial is prime?

(x4 + 3x2 - x2 - 3) (x4 - 3x2 - x2 + 3) (3x2 + x - 6x - 2) (3x2 + x - 6x + 3)

A polynomial is prime when it can't be factored anymore:

For

x4 + 3x2 - x2 - 3

It can still be factored into:

(x2 - 1) (x2 + 3)

For

x4 - 3x2 - x2 + 3

It can still be factored into:

(x2 - 3) (x2 - 1)

For

3x2 + x - 6x - 2

It can still be factored into:

(3x + 1) (x - 2)

For

3x2 + x - 6x + 3

It can't be factored anymore using real number.

Therefore, the last option is the prime polynomial