A man blows a whistle as he approaches a crossing. A few moments later he hears an echo of a whistle. A man hears the echo of the whistle because of:
A) Reflection
B) Refraction
C) Constructive Interference
D) Destructive Interference
E) Doppler effect

Hint: By concept of echo we know echo is sound heard when a sound is reflected from an obstacle when original sound has ceased. Example: When on mountains we hear echo of our own sound due to larger distance between mountains.

Complete step by step answer:
As we know the concept of echo is the reflection of sound when the original sound is reflected from a distant obstacle from a larger distance. As on mountains when we create a sound like our name we hear echo of our name for some time due to larger distances between mountains.
As in above example when man blows a whistle when he approaches a crossing and a few moments later he heard echo of sound that he had created, the sound of whistle created by man has been reflected from an obstacle across the crossing or something else, so the reason behind the echo of sound is reflection.
And also echo occurs when a sound wave reaches the ear more than seconds after the original sound has heard. And if the total time between the two sound waves is more than seconds then the feeling of the first sound will have died out and the second sound wave would be heard as the second sound rather than lengthening of the first sound wave.

Note: Remember that don’t mix this concept with Doppler effect as echo is reflection of sound wave from a distant obstacle placed at some distance and Doppler effect is change in frequency of sound waves depending on the motion of two observers.