Question

mention the condition for an echo to be heard clearly

Answer 1

The condition necessary is that the object to be sound will be reflected is to be 17 meters aways and the sound should be loud

Answer 2

The condition for an echo to be heard clearly are:

1) The distance between the person and the obstacle should around 16.5 m as the brain can only differentiate two successive sounds with 0.1 second time difference.

Echo => 2 x distance/time = speed of sound
                                 2x/0.1=330 m
                                      2x = 33  m
                                        x = 16.5 m

2) The room or the place should be quiet to hear the echo clearly.

Hope this helps......