Now wavelength of the sound waves $\lambda=\frac{v}{\nu}=\frac{330}{660}=0.5 \mathrm{m}$
Stationary waves will be formed due to the interference between the incident and the reflected wave and the displacement node is formed at the reflecting surface.
Thus the shortest distance between the wall (node) and the point where the air
particles have maximum amplitude of vibration (antinode) $d=\frac{\lambda}{4}=\frac{0.5}{4}=0.125 \mathrm{m}$
$(A)$ $u=0.8 v$ and $f_5=f_0$
$(B)$ $u=0.8 v$ and $f_5=2 f_0$
$(C)$ $u=0.8 v$ and $f_5=0.5 f_0$
$(D)$ $u=0.5 v$ and $f_5=1.5 f_0$