Final answer:
It is not permissible to keep the front access door of a bus open while it is moving under any normal circumstances for safety and regulatory reasons. The correct answer is d. None.
Step-by-step explanation:
Under normal circumstances, it is generally not permissible to keep the front access door of a bus open when moving. This applies to all cases whether there is less than 50 m between bus stops, when traveling at a low speed, or in very hot temperatures. The correct answer to the question is d. None. Keeping the doors closed during movement is important for passenger safety and to ensure compliance with traffic regulations and the standard operating procedures of most transit agencies.