If you meet a stopped school bus with the lights flashing and the signal arm extended, you should:

Answer: C — Stop

When approaching a stopped school bus with the lights flashing and the signal arm extended, the correct course of action is to stop. This is because children may be crossing the street to get on or off the bus, and proceeding without stopping could result in a dangerous situation. Option A, proceeding at 15 mph, is incorrect because any speed above zero could still put children at risk. Option B, proceeding at normal speed while watching for children, is also incorrect as it does not emphasize the importance of coming to a complete stop. Option D, stopping and waiting for the bus to resume motion, is also incorrect as it does not account for the possibility of children crossing the street in front of the bus. Therefore, the best and safest option is to come to a complete stop when approaching a stopped school bus with flashing lights and an extended signal arm.