When approaching a stopped school bus with its red flashing lights on, what must you do?

Answer: A — Stop until the lights go off and the bus moves.

When approaching a stopped school bus with its red flashing lights on, it is crucial to stop until the lights go off and the bus starts moving. This is because children may be getting on or off the bus and may not be paying attention to their surroundings. Passing at a slow speed (Choice B) can still pose a danger to the children, as can changing lanes and passing quickly (Choice C). Honking your horn (Choice D) may startle the children and cause them to act unpredictably. It is important to always prioritize the safety of children when sharing the road with a school bus.