To hear a clear echo, the reflecting surface must be at a minimum distance of:

Explanation

To hear a clear echo, the reflecting surface must be at a minimum distance that allows the original sound wave to travel to the surface, reflect, and return to the listener without interference. The minimum distance for a clear echo is determined by the time it takes for the sound to travel to the surface and back.The formula for the minimum distance (d) for a clear echo is given by:d=v/2 ⋅tTo hear a clear echo, the reflecting surface must be at a minimum distance, which corresponds to the time it takes for sound to travel to the surface and back.Now, the reflecting surface must be at least twice the minimum distance for the sound to travel to the surface and return. Therefore, the minimum distance for a clear echo (d) is:d=v/2 ⋅t2d=v.tGiven that the speed of sound (v) is approximately 343 m/s, let's evaluate the options:2x10m=20m2x16.5m=33m2x33m=66m2x66m=132mAmong the options, the correct answer is:c. 33m