Among the given options, which one causes an echo?

Correct option is A

The correct answer is: (a) When sound is reflected
 An echo occurs when sound waves are reflected off a surface and return to the listener's ears after traveling a distance.
 The phenomenon of an echo is a result of sound waves bouncing off surfaces such as walls, cliffs, or mountains and traveling back to the listener's ears.
 The time delay between the original sound and its reflection determines the perceived echo effect, with larger distances resulting in longer delays.