The correct answer is D. Unable to proceed, Guard protect the train in rear.
A train whistle is a device used to alert people and other trains to the presence of a train. It is typically sounded by the locomotive engineer, or loco pilot, to warn of the train’s approach. The whistle is also used to communicate with other train crews and to signal to the train’s crew when to start, stop, or slow down.
The four short whistle sound is a standard signal used by train crews to indicate that the train is unable to proceed and that the guard should protect the train in the rear. This signal is typically used when the train has a mechanical problem or when there is a problem on the track ahead.
The other options are incorrect. Option A, Signal Approaching, is incorrect because the four short whistle sound is not used to indicate that a signal is approaching. Option B, Train out of control, is incorrect because the four short whistle sound is not used to indicate that the train is out of control. Option C, Train parting, is incorrect because the four short whistle sound is not used to indicate that the train is parting.