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

The Doppler Effect provides the equation for the calculation of apparent frequency:

f=fo[vo / (vo-vr) ]

where:

vo=source wave velocity

vr=relative speed between source and observer

f=apparent frequency

fo=source frequency

The velocity of the doppler wave is

v=λf

where λ is light wavelength. Hence,

v=λfo[vo / (vo-vr) ]

Based on the equation, we can say that wave velocity will always be defined by one and only one wavelength.

Therefore the answer is letter C.