Ans: C
$\begin{array}{rcl}
\dfrac{a+5b}{7a+2b} & = & \dfrac{1}{b+3} \\
(a+5b)(b+3) & = & 7a+2b \\
ab+3a+5b^2+15b & = & 7a+2b \\
ab-4a & = & -5b^2-13b \\
a(b-4) & = & -5b^2-13b \\
a & = & \dfrac{-5b^2-13b}{b-4} \\
a & = & \dfrac{5b^2+13b}{4-b}
\end{array}$
