Under what circumstances is it permissible to keep the front access door of a bus open when moving?

Answer: C — None

Keeping the front access door of a bus open while moving is not permissible under any circumstances. This is because it poses a safety risk to passengers and other drivers on the road. Option A When the bus is empty, is incorrect because even if the bus is empty, the door should still be closed for safety reasons. Option B: During short trips, is also incorrect because the duration of the trip does not change the safety concerns of having the door open while the bus is in motion. Option D: If going under 20 km/h, is incorrect because the speed of the bus does not eliminate the potential danger of an open door.