b
(b) On getting reflected from a rigid boundary the wave suffers
Hence if ${y_{incident}}$= $A\sin (\omega t - kx)$
then ${y_{reflected}} = (0.8A)$ $\sin \left\{ {\omega t - k( - x) + \pi } \right\}$
$ = - 0.8A\sin (\omega t + kx)$ an additional phase change of $\pi $.