What indicates that a school bus is about to stop?

The correct choice indicates that the yellow caution lights being activated is a warning to other drivers that the bus is preparing to stop to pick up or drop off children. These yellow lights serve as a signal that drivers should slow down and be prepared to stop, as the situation is changing.

This stage is crucial for safety, as it alerts drivers to the potential for children to be near the bus and encourages them to be vigilant. The yellow lights are essentially a precautionary measure, providing advanced notice before the bus engages its red flashing lights and extends its stop sign, which would require vehicles to come to a complete stop.

In contrast, flashing red lights and the deployment of the stop sign are definitive signals that require drivers to halt, while a physical signal from the driver, such as waving their hand, is less standardized and may not be as easily recognized by all drivers.