Ans: B
Let $w=\dfrac{kx^2}{y^3}$, where $k$ is a non-zero constant. Hence, we have
$\begin{array}{rcl}
w & = & \dfrac{kx^2}{y^3} \\
k & = & \dfrac{wy^3}{x^2} \\
\dfrac{1}{k} & = & \dfrac{x^2}{wy^3}
\end{array}$
