Question

if the source is moving away from the observer , then the apparent frequency.........

a) will increase
b)will remain the same
c)will be zero
d) will decrease

Answer 1

d) will decrease ...

Answer 2

Answer : (d) decreases

It's depends upon Doppler effect .
According to Doppler effect
$\bold{\nu=\nu_0[\frac{v\pm v_0}{v\pm v_s}]}$
Here ν is apparent frequency , ν₀ is initial frequency ,V₀ is velocity of observer and Vs is velocity of source

Now, when source is moving away from the observer and observer is in rest
Means V₀ = 0 , for towards , we should use V + Vs
Then, ν = v₀( V + 0)/(V + Vs)
ν = ν₀V/(V + Vs) means ν < ν₀

Hence, apparent frequency decreases