c
(c) Given $y = 5\sin \frac{{\pi x}}{3}\cos 40\,\pi t$
Comparing with $y = 2a\cos \frac{{2\pi vt}}{\lambda }\sin \frac{{2\pi x}}{\lambda }$$ \Rightarrow \lambda = 6\,cm.$
$\therefore$ The separation between adjacent nodes $ = \frac{\lambda }{2} = 3\,cm.$