What sign warns you that you must slow down?

The correct answer is the yellow diamond warning signs. These signs are specifically designed to alert drivers to potential hazards or changes in driving conditions. When you see a yellow diamond sign, it indicates that you should be prepared to slow down and proceed with caution. These signs can indicate various warnings, such as curves in the road, pedestrian crossings, or other situations where reduced speed is necessary for safety.

In contrast, a red stop sign indicates a complete stop is required and does not simply suggest that one should slow down; it mandates that drivers come to a full stop before proceeding. Green directional signs provide information about direction and distance to various destinations but do not require a change in speed. Blue informational signs offer guidance related to services and amenities like gas stations or hospitals, which may indicate the presence of facilities but do not instruct drivers to alter their speed in the way that warning signs do.