Question

Which of the following describes a condition in which an individual would not hear an echo?

Answer 1

The answer is C. A sound reaches the ear and the reflected wave reaches the ear less than 0.1 second later

Answer 2

If the echo (the reflected sound) reaches your ear less than about
0.1 second after the original sound, your brain doesn't separate them,
and you're not aware of the echo even though it's there.

If the echo comes from, say, a wall, 0.1 second means you'd have to be
about  17 meters away from the wall.  If you're closer than that, then the
echo reaches you in less than 0.1 second and you're not aware of it.

A. 30 meters . . .
     No.  You hear that echo easily

B.  you're standing within range of both sounds . . .
     No. You hear that echo easily, if you're at least 17 meters from the wall.

C.  less than 0.1 second later . . .
     That's it.  The echo is there but your brain doesn't know it.

D.  21.5 meters
     No.  You hear that echo easily.