Answer
$f_n(x) = x^{2^{n+1}}$
Work Step by Step
We can evaluate the the first elements of the sequence:
$f_0(x) = x^2$
$f_1(x) = (x^2)^2 = x^4$
$f_2(x) = (x^4)^2 = x^8$
$f_3(x) = (x^8)^2 = x^{16}$
etc...
We can see that $f_n(x) = x^{2^{n+1}}$