When a school bus is loading or unloading passengers with red signal lights flashing, motorists approaching from behind must stop at least how far away?

Answer: C — 20 metres

When a school bus is loading or unloading passengers with red signal lights flashing, motorists approaching from behind must stop at least 20 metres away. This is because this distance allows enough space for the driver to see the flashing signal lights and safely stop behind the school bus. Option A, B, and D may seem like suitable distances, but they are not enough since it may cause the driver to suddenly brake and potentially cause a collision. Therefore, option C is the most appropriate choice.