To hear a distinct echo in air, the reflecting surface must be at a minimum distance of about:

The correct answer is B, about 17 metres. The sensation of a sound persists in the human brain for roughly one tenth of a second, so a reflected sound must arrive at least that much after the original to be heard as a separate echo. In that tenth of a second sound in air covers about 34 metres, but the sound must travel to the surface and back, so the surface itself must be at least half of that away, roughly 17 metres.

Option A, 1.7 metres, is ten times too small and would give a reflection arriving in about one hundredth of a second, far too soon to be heard separately. Option C, 34 metres, is the total path length travelled by the sound rather than the distance to the wall, and is the most common error in this question. Option D, 170 metres, is ten times too large; an echo is heard well before the surface is that far away.