When driving in California, what must you do when approaching a stopped school bus with its red lights flashing?

Answer: C — Reduce your speed and stop your vehicle

When approaching a stopped school bus with its red lights flashing in California, it is important to reduce your speed and come to a complete stop. This is because it is illegal to pass a school bus when its red lights are flashing and children may be getting on or off the bus. Options A and B are incorrect because they advise increasing or maintaining speed, which is dangerous and illegal in this situation. Option D is incorrect because it suggests waiting for the bus to move forward, which is also not allowed. A complete stop is necessary to ensure the safety of the children on the bus.