If the driver does not receive all right signal from the guard, he must call by giving. A. Two sharp whistles B. One long one short C. On long D. None of these

The correct answer is A. Two sharp whistles.

The driver must give two sharp whistles if he does not receive the all-right signal from the guard. This is to alert the guard that there is a problem and that the train should not proceed.

Option B is incorrect because one long and one short whistle is the signal to proceed. Option C is incorrect because one long whistle is the signal to stop. Option D is incorrect because none of these options are correct.