What must you do when you approach a school bus with its flashing red lights?

Answer: B — Stop and wait until lights are off

When approaching a school bus with flashing red lights, it is important to stop and wait until the lights are turned off. This is necessary because it indicates that children are either getting on or off the bus, and it is illegal to pass a school bus with flashing red lights in most places. Option A, reducing the speed limit, is not sufficient as it does not ensure the safety of the children. Option C and D are incorrect as passing on the left side or maintaining the same speed of travel can put children at risk and is against the law. It is important to always follow traffic laws and be cautious when driving near a school bus.