Final answer:
School bus warning lights must flash at all times when passengers are boarding and exiting to ensure safety, irrespective of the time of day or school hours.
Step-by-step explanation:
When passengers are boarding and exiting the vehicle, school bus warning lights must flash b) At all times. This is a safety measure that ensures other drivers and pedestrians are alerted to the presence of children entering or leaving the bus. It is not limited to any specific time of day or school hours. Whether it's daylight or nighttime, or during any time the bus is stopped to let passengers on or off, the warning lights should be flashing. This helps prevent accidents and ensures that children are provided with a safe environment while they are near the school bus.