A person clapped his hands near a cliff and heard an echo after 5s. What is the distance of the cliff from the person if the speed of the sound is taken as 346 m/s?

Given,
Speed of sound, v = 346 m/s
Time is taken for hearing the echo, t = 5 s
Distance travelled by the sound: velocity  x time
Distance = v x t = 346 x 5 = 1730 m

The echo has to travel twice the distance between the cliff and the person. 

 Hence, the distance between the cliff and the person$\frac{1730 \mathrm{m}}{2}= 865 \mathrm{m}$.

Therefore, 865 m is the correct response.