**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**