d
(d)

\(y=0.2 \sin 0.8 x \cos 3000 t\)

The distance between any two points of minimum or maximum displacement is simply wavelength \(\left(\frac{\lambda}{4}\right)\) of the wave.

\(k=\frac{2 \pi}{\lambda}=0.8\)

\(\text { or } \lambda=\frac{2 \pi}{0.8}\)

\(\therefore \lambda=\frac{5 \pi}{2} m \Rightarrow \frac{\lambda}{4}=\frac{5 \pi}{8} \,m\)

Hence answer must be \((d)\).