**Concept: The question tests on the area of Rates(Time and Work)**

**Solution:**

LCM of 60 and 40 is 120

Let 120 units of work be done

Work done by the Man working alone = 120/60 = 2 units/hour

Work done by the Man + Son working together = 120/40 = 3 units/hour

Work done by the son working alone = 3 - 2 = 1 unit/hour

Therefore to do 120 units, he would require 120/1 = 120 hours**Option D**