What is called the phenomenon of prolongation of sound​

It is called reverberation.

Explanation:

The phenomenon of prolongation of sound is the process in which sound produced is reflected from plane surfaces in a continuous manner, which leads to reverberation of sound waves. It can also be referred to as a condition of continuous echo production.

It occurs when sound produced get reflected from a hard plane surface, and in the process hits another surface and another reflection occurs. This continuous reflection leads to prolongation of sound.

The sound produced is heard for some specific period of time before it dies down or get absorbed finally.

think its the third option

its c i just checked it