The correct answer is A. 0-0.
The code of engine whistle during signal passing at ON or in defective condition on proper authority information by the Loco Pilot to Guard is 0-0. This means that the signal is at ON and the loco pilot has received proper authority from the guard to pass the signal.
The other options are incorrect because they do not correspond to the correct code for the situation described. Option B, -0-, is the code for a signal at danger. Option C, –0, is the code for a signal at caution. Option D, —, is the code for a signal at all clear.
It is important to know the correct code for engine whistle during signal passing at ON or in defective condition on proper authority information by the Loco Pilot to Guard in order to ensure the safety of train operations.