College Algebra (11th Edition)

A. ($x^{2}$ - 1)($x^{2}$ + 1)
$x^{4}$ - 1 is a difference of squares, for which $x^{2}$ - $y^{2}$ = (x + y)(x - y), so the expression factors to ($x^{2}$ - 1)($x^{2}$ + 1). We can also use FOIL to check.