The correct answer is: D. Unable to proceed
The three short whistle sounds by the loco pilot means that the train is unable to proceed. This could be due to a number of reasons, such as a mechanical failure, a signal problem, or a collision. When the loco pilot sounds the three short whistle sounds, it is a warning to other trains and personnel that the train is not able to continue on its journey.
The other options are incorrect. Option A, “Signal Approaching”, is incorrect because the three short whistle sounds are not used to indicate that a signal is approaching. Option B, “Train out of control”, is incorrect because the three short whistle sounds are not used to indicate that a train is out of control. Option C, “Train parting”, is incorrect because the three short whistle sounds are not used to indicate that a train is parting.