Wave length of light used in an optical instrument are $ \lambda_1=4000^{\circ}A$ and $ \lambda_2=5000^{\circ}A$ then ratio of their respective resolving power .(corresponding to $\lambda_1$ and $\lambda_2$) is

Question

$\begin{array}{1 1} 16:25 \\ 9:1 \\ 4:5 \\ 5:4 \end{array} $