Total time taken by the man to travel from A to D = 16 hr and total distance traveled = 36 km.

The time that he would have taken had he not rested in between will be (16-x-2x) = (16-3x).

But, this time should be equal to the addition of the times that he takes to travel individual segments. This is given as:

12/x + 12/2x + 12/4x = 21/x

Therefore, 21/x = 16-3x

x = 3 or 7/3

**Option A is the Correct Answer.**