(d) Resolving power of an optical instrument is inversely proportional to $\lambda\ i.e.,\ RP \propto \frac{1}{\lambda}\ $
$\therefore\frac{Resolving\ power\ at\ \lambda_{1}}{Resolving\ power\ at\ \lambda_{2}} = \frac{\lambda_{2}}{\lambda_{1}} = \frac{5000}{4000} = 5:4$