How will you know when a school bus is preparing to stop to pick up children?

Answer: C — stop sign arm

When a school bus is preparing to stop to pick up children, the driver will activate the stop sign arm which extends from the side of the bus. This is a universal signal for other drivers to come to a complete stop to allow children to safely cross the road. Choice A, flashing yellow lights, indicates that the bus is preparing to stop and warning other drivers to slow down, but not yet picking up children. Choice B, flashing purple lights, is not a standard signal for school buses and is therefore incorrect. Choice D, flashing red lights, are activated when the bus has come to a complete stop and children are entering or exiting the bus. This is different from the stop sign arm which is activated before the bus comes to a stop. Therefore, option C is the correct answer as it specifically addresses the signal used by school buses to indicate a pick-up stop for children.