When should you turn on your upper alternating red signal lights when stopping a school bus to load or unload passengers?

Answer: C — Before you stop

The other options are incorrect because turning on the upper alternating red signal lights after you stop (option A) or right before opening the doors (option B) would not give other drivers enough warning that you are about to stop to load or unload passengers. Turning on the lights as you approach the bus stop (option D) is also incorrect because you should be focusing on safely stopping the bus at the designated location, rather than trying to turn on the signal lights while still moving. As such, turning on the upper alternating red signal lights before you stop (option C) is the most appropriate and safe action to take.