Concept: The question tests on the basics of Rates(Time and Work)
Solution:
There are 2 ways we can go about solving this
(1) By the concept of Relative Speed, when 2 objects are moving in the same direction, the net effect is the difference between the speeds.
Let the policeman's speed = x m/s.
Then Time = Distance / Relative Speed
8 = 40 / (x - 10)
8x - 80 = 40
8x = 120
x = 15 m/s
(2) In 8 seconds the thief has run 8 * 10 = 80 m. Therefore the total distance run by the thief = 80 + 80 = 160 m
Let the speed of the policeman = x m/s. The distance covered by the policeman = 8x
After 8 seconds the policeman is 40 m behind the thief, which means the distance covered by the policeman = 160 - 40 = 120m
8x = 120
x = 15 m/s
Option C