Which of the following is an example of a warning sign?

Answer: D — School crossing

Warning signs tell you to expect a condition or hazard ahead so you can slow down and be prepared to act. The school crossing sign is the classic example of this type because it specifically warns that children may be crossing near a school, signaling you to reduce speed, scan for pedestrians, and be ready to stop if needed. The other signs also warn about upcoming conditions (stop ahead, railway crossing ahead, yield ahead), but the school crossing sign directly highlights a pedestrian hazard in a school zone, making it the clearest illustration of a warning sign.