The correct option is D v−uv+u.vf
Given, velocity of sound = v
velocity of wall = u
then, relative speed of sound w.r.t. wall before reflection= v+u
Let, apparent frequency of the wall is f′.
Then, f′=(v+uv)f .......(1)
Relative speed of sound w.r.t. wall after reflection = v−u
∴ wavelength of sound reflected by wall =v−uf′
Putting f′ value from equation (1)
Wavelength of sound reflected by wall =v−uv+u×vf
Hence, option (4) matches correctly.