The correct answer is D. 15 minutes.
The guard of a suburban train shall sign on 15 minutes before the actual departure of the train. This is to ensure that the train is ready to depart on time and that all passengers are on board. The guard is responsible for checking the train’s safety and for ensuring that all passengers have a valid ticket.
Option A is incorrect because it is too long. The guard does not need to sign on 45 minutes before the train’s departure.
Option B is incorrect because it is too short. The guard needs to sign on at least 15 minutes before the train’s departure to ensure that everything is ready.
Option C is incorrect because it is not a valid option. The guard does not need to sign on 40 minutes before the train’s departure.