Concept: The question tests on the basics of Rates (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