(a) The speed of sound increases with the increase of temperature of the medium. The speed of sound in air increases by 0.61 m/s when the temperature is increased by 1° C. Hence, option D is the correct answer.

(b) Let the speed of sound be v

Distance of the man from the cliff is d

Time taken to hear the echo is 4 s

Velocity of the sound wave to reach the person before he moved closer

Now, after moving closer the distance of man is (d – 84) m

Time elapsed before the man hear the echo again after moving closer to the cliff is 3.5 s

The speed of the sound remains the same in either case. Thus,