Final answer:
The emergency door on a bus must be closed when the bus is operating to ensure passenger safety. The red light indicator and use for fresh air are not primary functions of the emergency door.
Step-by-step explanation:
The emergency door on a bus plays an essential role in passenger safety, and there are specific regulations about how it should be maintained and used. In answer to the student's question, A. It must be closed when operating the bus is the correct option. The emergency exit door of a bus must always be closed when the bus is in operation. This is crucial to ensure the safety of the passengers inside the vehicle. While option B, which states it must always have a red door light on, is not a universal requirement, the red light is commonly used to indicate that the emergency door is not securely closed. Option C, which suggests it must be free to open for fresh air, is incorrect as the primary purpose of the emergency door is for safe exit during emergencies, not for ventilation under normal circumstances. So, the only correct statement regarding the emergency door on a bus is that it must be closed while the bus is in operation.