Answer:
Option A
Explanation:
When the observer is moving towards the source and source is stationary , then the apparent frequency is given as,
n′=n(v+v0v) ..........(i)
When the observer is moving away from the source and the source is stationary , then the apparent frequency is given as,
n″=−n(v−v0v) .......(ii)
From Eqs .(i) and (ii) , we get
n′−n″=nv(v′+v0−v+v0)
= 2nv0v