15. The highest positive integer power of 2 that is a factor of $13^{4}-11^{4}$ is ( ).
(A) 8
(B) 16
(C) 32
(D) 64
(E) 128
15. C.

Notice,
$$
\begin{array}{l}
13^{4}-11^{4} \\
=\left(13^{2}+11^{2}\right)(13+11)(13-11) \\
=290 \times 24 \times 2=2^{5} \times 3 \times 145 .
\end{array}
$$

Therefore, the answer is $2^{5}=32$.