11. Given that the ratio of the length to the width of a rectangle is $4: 3$, and the length of the diagonal is $d$. If the area of the rectangle is expressed as $k d^{2}$, then the value of $k$ is ( ).
(A) $\frac{2}{7}$
(B) $\frac{3}{7}$
(C) $\frac{12}{25}$
(D) $\frac{16}{25}$
(E) $\frac{3}{4}$
11. C.

Let the length of the rectangle be $4 x$, and the width be $3 x$. Then $d=5 x$. Thus, $x=\frac{d}{5}$. Therefore, $S=3 x \cdot 4 x=12 x^{2}=\frac{12}{25} d^{2} \Rightarrow k=\frac{12}{25}$.