b
(b) Phase difference between two successive crest is $2 \pi$.
Also, phase difference $(\Delta \phi )$= $\frac{{2\pi }}{T}$ time interval $(\Delta t)$
==> $2\pi = \frac{{2\pi }}{T} \times 0.2 \Rightarrow \frac{1}{T} = 5{\sec ^{ - 1}} \Rightarrow n = 5\,Hz$