Answer
$(x-1)(x+1)(x^2+1)$
Work Step by Step
The polynomial $x^4-1$ is a difference of two squares. That is, by using the fact that $a^2-b^2=(a-b)(a+b)$, we have
$$
x^4-1=(x^2-1)(x^2+1)=(x-1)(x+1)(x^2+1)
.$$
Moreover, the polynomial $x^2+1$ is a prime polynomial.
