The correct answer is D. At the time of on duty.
The loco pilot must set his watch to the station master’s watch at the time of on duty. This is to ensure that the loco pilot and the station master are on the same time and that there is no confusion about the time of departure or arrival of the train.
Option A is incorrect because the loco pilot must set his watch to the station master’s watch at the time of on duty, not at 8.00 Hrs.
Option B is incorrect because the loco pilot must set his watch to the station master’s watch at the time of on duty, not at 16.00 Hrs.
Option C is incorrect because the loco pilot must set his watch to the station master’s watch at the time of on duty, not at both 8.00 Hrs and 16.00 Hrs.