The minimum distance (in metres) required to hear an echo is 17 metres. This is because the speed of sound in air is 343 metres per second, so it takes 0.05 seconds for sound to travel 17 metres. The human ear can detect a time difference of up to 0.01 seconds between the original sound and the echo, so if the sound source is closer than 17 metres, the echo will be too close to the original sound to be heard as a separate sound.
Option A (10 metres) is incorrect because the sound will not have had enough time to travel back to the listener by the time the original sound has stopped.
Option B (13 metres) is incorrect because the sound will have just had enough time to travel back to the listener by the time the original sound has stopped, but the echo will be very faint.
Option C (17 metres) is the correct answer because the sound will have had enough time to travel back to the listener by the time the original sound has stopped, and the echo will be loud enough to be heard.
Option D (21 metres) is incorrect because the sound will have had too much time to travel back to the listener by the time the original sound has stopped, and the echo will be too faint to be heard.