A driver in a stationary car blows a horn which produces monochromatic sound waves of frequency 1000 Hz normally towards a reflecting wall. The wall approaches the with a speed of 3.3 ms^-1 ( velocity of sound is 333 ms^-1)

First by using doppler’s effect we can obtain the frequency of sound received by the wall considering it is a moving observer then we consider it as a moving source of this frequency and then we can find the frequency received by the driver as an observer. The analysis will result in option (A) and (C) correct
${\mathrm{n}}^1=\mathrm{n}\left(\frac{\mathrm{V}+{\mathrm{V}}_{\mathrm{\omega }}}{\mathrm{V}-{\mathrm{V}}_{\mathrm{\omega }}}\right)$
% change in frequency is
$\frac{\mathrm{\Delta n}}{\mathrm{n}}\times 100=\frac{(1020-1000)}{1000}\times 100=2$