17. On each side of the square $ABCD$, equilateral triangles are constructed outward, and $E, F, G, H$ are the centers of these equilateral triangles, respectively. Then the ratio of the area of square $EFGH$ to the area of square $ABCD$ is ( ).
(A) 1
(B) $\frac{2+\sqrt{3}}{3}$
(C) $\frac{\sqrt{2}+\sqrt{3}}{2}$
(D) $\sqrt{2}$
(E) $\sqrt{3}$
17. B.

As shown in Figure 3, establish a Cartesian coordinate system. Let $A(1,1), B(-1,1), C(-1,-1), D(1,-1)$. Then $E\left(1+\frac{\sqrt{3}}{3}, 0\right), F\left(0,1+\frac{\sqrt{3}}{3}\right)$. Therefore, $\frac{S_{\text {square } E F G H}}{S_{\text {square } A B C D}}=\left[\frac{\left(1+\frac{\sqrt{3}}{3}\right) \sqrt{2}}{2}\right]^{2}=\frac{2+\sqrt{3}}{3}$.