Concept: The question tests on the basics of  Rates(Speed and Time)

Solution:

Without stoppage the train travels 50 km in 60 minutes. Therefore it travels 1 km every 60/50 = 6/5 minutes

With stoppage it travels 40 km in 1 hour.

To travel 40 km, it requires 40 * (6/5) = 48 minutes

Therefore his stoppage time is 60 - 48 = 12 minutes/h


Option d