b
(b) Since the point $x = 0$ is a node and reflection is taking place from point $x = 0.$
This means that reflection must be taking place from the fixed end and hence the reflected ray must suffer an additional phase change of $\pi$ or a path change of $\frac{\lambda }{2}$.
So, if ${y_{{\rm{incident}}}} = a\cos (kx - \omega \,t)$
==> ${y_{{\rm{reflected}}}} = a\cos ( - kx - \omega \,t + \pi )$$ = - a\cos (\omega \,t + kx)$