25. From a square with a side length of 5, cut out a small square with a side length of 1 from each of the four corners. In the remaining shape, a square with an area of $(\quad)$ can be inscribed.
(A) 9
(B) 12.5
(C) 15
(D) 15.5
(E) 17
25. C.

As shown in Figure 5, the required part is the square $A B C D$. By symmetry, we have
$$
\begin{array}{l}
S_{\text {square } A B C D}=S_{\text {square } A^{\prime} B^{\prime} C^{\prime} D^{\prime}}+4 S_{\triangle A A^{\prime} B^{\prime}} \\
=(5-1 \times 2)^{2}+4 \times \frac{1}{2} \times(5-1 \times 2) \\
=15 .
\end{array}
$$