In which one of the following locations can a person hear the echo of a sound?
*a. *less than 16.6 meters in front of the reflecting surface
*b. *at least 33.2 meters behind the reflecting surface
*c. *less than 33.2 meters behind the reflecting surface
*d. *at least 16.6 meters in front of the reflecting surface

If you are at least roughly 16.6 meters or so from the
reflecting surface, then you can hear the echo.  If you're
closer than that, there's still an echo, but the delay between
the original sound and the echo is so short that your ear/brain
mechanism can't separate them.

The speed of sound in typical air is around 340 meters per
second.  So somewhere around 16.5 - 17 meters of physical
separation results in 1/10 of a second delay until the echo. 
That's considered to be the minimum perceptible delay for
an echo.