The correct answer is: 15.
The SM (Station Master) must take action when a train carrying passengers does not arrive within 15 minutes plus running time. This is because the SM is responsible for the safety of the passengers and must ensure that they are not stranded at the station. If a train is delayed, the SM will need to make arrangements for the passengers to be taken to their destination.
Option A: 10 minutes is not the correct answer because it is not enough time to take action. The SM needs to have enough time to make arrangements for the passengers to be taken to their destination.
Option B: 20 minutes is not the correct answer because it is too much time. The SM needs to take action as soon as possible to ensure the safety of the passengers.
Option C: 15 minutes is the correct answer because it is enough time for the SM to take action and make arrangements for the passengers to be taken to their destination.