An echo is a sound caused by the [[1]] of sound waves from a surface back to the listener.

• An echo is a sound caused by the reflection of sound waves from a surface back to the listener. 
• Echo is a reflection of the sound that comes to the listener with a pause after the direct echo. 
• The delay is directly proportional to the distance from the source and the listener to the reflecting surface.

Therefore, 'reflection' is the correct response.