Ans : (c)
$ \lambda\: \sim\: \large\frac{1}{p}$
$ \Rightarrow \large\frac{\Delta p}{p} = -\large\frac{ \Delta \lambda}{ \lambda}$
$ \Rightarrow |\large\frac{ \Delta p}{p} | = | \large\frac{ \Delta \lambda}{ \lambda}|$
$\Rightarrow \large\frac{p_{\circ}}{p} = \large\frac{0.25}{100} = \large\frac{1}{400}$
$ \Rightarrow p = 400p_{\circ}$