Answer : Option C

Explanation :

Sound is a sequence of waves of pressure that propagates through compressible media such as air or water. (Sound can propagate through solids as well, but there are additional modes of propagation). Sound that is perceptible by humans has frequencies from about 20 Hz to 20,000 Hz. In air at standard temperature and pressure, the corresponding wavelengths of sound waves range from 17 m to 17 mm. During propagation, waves can be reflected, refracted, or attenuated by the medium. Now if we consider these cases, then the train which is arriving towards us having whistles of higher pitch because it propagates through a medium which is coming towards us but the train which is leaving propagating through a medium moving further away from the listener and thus produced whistle of lower pitch.