If you mean x^4, or x to the 4th power , when you write x4, then none of the answers make sense, since x^4 + y^4 cannot be factored.
If x^4 - y^4 is in fact intended, then the answer is A,
since x^4 - y^4 = (x^2 + y^2)(x^2 - y^2). So,
x2 + y2
=(x^4 - y^4) / (x^2 - y^2)
=[(x^2 + y^2)(x^2 - y^2)] / (x^2 - y^2)
x^2 + y^2