When echo is heard
A) If the time interval between original sound and reflected sound is more than \[\dfrac{1}{10s}\]
B) If the time interval between original sound and reflected sound is less than \[\dfrac{1}{10s}\]
C) If the time interval between original sound and reflected sound is less than \[\dfrac{1}{120s}\]
D) If the time interval between original sound and reflected sound is \[\dfrac{1}{120s}\]

Hint: Echo is produced due to reflection of sound. If a sound wave travelling cannot pass through an obstacle it reflects back towards the source which is producing it causing an echo. To hear an echo there are conditions which we will discuss here briefly and from that we will get our answer.

Complete step by step answer:
Echo is a phenomenon in which we hear a sound wave repeatedly due to reflection of the sound wave from an obstacle. For example, the echo formed by the bottom of the well or by the walls of an empty room or the reflected waves from the hills.
To hear an echo there are conditions like the well should be deep enough to form an echo or the room should be vacant and spacious. Similarly, a human ear cannot differentiate between the original sound wave (or the wave produced by a source) and the reflected sound wave if the time duration is less than 0.1seconds or \[\dfrac{1}{10s}\]. If the time interval is less than 0.1 seconds then the echo formed is not audible for the humans.

So, the correct answer is “Option A”.

Note: The time interval calculated for the sound wave is the time taken by the sound wave to travel back and forth. For example, if we are considering the well, then the distance travelled by sound wave will be twice. First when it reaches the bottom and then the distance travelled by the reflected wave which is received by the source.