Explanation

(b) As is clear from figure of question
$l = \frac{\lambda_{p}}{4},\ \lambda = 4l,\ n_{p} = \frac{v}{\lambda_{p}} = \frac{v}{4l}$
$l = \frac{\lambda_{q}}{2},\lambda_{q} = 2l,\ n_{q} = \frac{v}{\lambda_{q}} = \frac{v}{2l}$
$l = \lambda_{r},\ \lambda_{r} = l,\ n_{r} = \frac{v}{\lambda_{r}} = \frac{v}{l}$
$l = \frac{3\lambda_{s}}{4},\ \lambda_{s} = \frac{4l}{3},\ n_{s} = \frac{v}{\lambda_{s}} = \frac{3v}{4l}$
$\therefore n_{p}:n_{q}:n_{r}:n_{s} = \frac{v}{4l}:\frac{v}{2l}:\frac{v}{l}:\frac{3v}{4l} = 1:2:3:4$