The correct answer is: B. 30 minutes before the actual departure of the train.
The guard of a goods train is responsible for the safe and efficient operation of the train. They must ensure that the train is in good condition and that all passengers and crew are safe. The guard must also sign on at least 30 minutes before the actual departure of the train to ensure that they are ready to depart on time.
Option A is incorrect because the guard must sign on at least 30 minutes before the actual departure of the train, not 45 minutes.
Option C is incorrect because the guard must sign on at least 30 minutes before the actual departure of the train, not 40 minutes.
Option D is incorrect because the guard must sign on at least 30 minutes before the actual departure of the train, not 35 minutes.