What must be done when passing a stopped school bus with flashing red lights?

Answer: A — Come to a complete stop

When passing a stopped school bus with flashing red lights, it is required to come to a complete stop. This is important to ensure the safety of children crossing the road, as well as to follow traffic laws. B is incorrect because simply reducing speed is not sufficient in this situation. C is incorrect because changing lanes may not be a viable option or could further put children at risk. D is incorrect because a complete stop is necessary when passing a stopped school bus with flashing red lights.