Correct Answer - Option 3 : 2 hour

Three persons take 20, 30 and 40 minutes to complete one round.

L.C.M of 20, 30 and 40 minutes is 120 minutes.

120 minutes = 2 hours

So, again after 120 minutes or 2 hours, they will meet again at that point.

Hence, ‘2 hours’ is the correct answer.