What is meant by an echo? What is the condition necessary for an echo to be heard distinctly?

Hint: To be able to answer this question we should know what reflection of sound is, and in the reflection of sound , when the sound moves from one medium to alternative then it strikes the surface of medium and bounds back in the similar medium. So by using this information one can easily answer this question.

 Complete Step by Step Solution: An echo can be defined as a reflection of sound that arrives at the listener with a delay after the direct sound. The time it takes to arrive is directly proportional to the distance of the reflecting surface from the source and the listener.
The necessary conditions for hearing an echo can be,
The distance between the sound source and the place where it is reflected from must not be less than ${\text{17m}}$ and the period between hearing the original sound and its echo should not be less than $0.1$ of a second.
The human ear cannot distinguish between two successive sounds if the time gap between them is less than $0.1\sec $ , and the place from where it is being reflected must be presented to hear the echo such as the walls, the mountains, or the water bodies.
The speed of sound while traveling in air is ${\text{340m/s}}$ , so the distance traveled by the sound and its echo in $0.1\sec $ would be ${\text{34m}}$ . The distance traveled by the sound is ${\text{17m}}$ from the sound source to the place where it will be reflected from and ${\text{17m}}$ from the same place to the ear.
When the distance between hearing the actual sound and its reflection is in the multiples of ${\text{17m}}$ , only the last two or three phases of the whole sound can be heard as an echo.

 Note: We should rightly use the value of the speed of sound here then we can get the right value for the minimum distance at which the reflecting surface has to be otherwise the distance we get will be wrong and the answer will become wrong. So we have to be careful while putting the numerical values.