Concept: The question tests on Rates (Time Speed Distance)

Solution:
Without stoppage he travels 18 km in 60 minutes. Therefore he travels 1 km every 60/18 = 10/3 minutes

With stoppage he travels 12 km in 1 hour.

To travel 12 km, he requires 12 * (10/3) = 40 minutes

Therefore his stoppage time is 60 - 40 = 20 minutes

Option D