The velocity of the wave involved in the doppler effect:
a. will always be a constant
b. depends upon the medium in which it occurs
c. will always be defined by one and only one wavelength
d. will always be defined by one and only one frequency

b. depends upon the medium in which it occurs

Explanation:

When source of sound and observer moves relative to each other then in that case the relative motion between them will change the frequency that is observed by the observer.

It is known as Doppler's effect. In that case the velocity of wave depends on the medium in which wave travels.

As we know that wave speed is given as

so here as the medium of the wave changes the speed of wave also be different so here correct answer will be

b. depends upon the medium in which it occurs