To hear a distinct echo the time interval between the original sound and the reflected one must be at least _____.

• A repeated sound caused by the reflection of the sound waves from the surface is called an echo.
• The sensation of sound persists in our ears for about 0.1 seconds. 
• To hear a distinct echo, the time interval between the original and reflected sound must be greater than or equal to 0.1 seconds.

Therefore, '0.1 s' is the correct response.