What color is often used on signs that indicate a warning about a hazard ahead?

Answer: C — Yellow

The use of yellow for warning signs is standardized in traffic safety to alert drivers to potential hazards or changes in road conditions. This color is highly visible and attention-grabbing, which is critical for ensuring that drivers can quickly recognize and respond to warnings about dangers such as road work, curves, or intersections. Yellow signs are designed to convey caution, prompting drivers to reduce speed or be prepared for unexpected situations ahead. In contrast, the other colors listed serve different purposes; for example, green is commonly used for guide signs indicating directions or distances, red signifies stop or prohibition, and blue typically indicates services or information for travelers. Each color in the traffic sign system plays a critical role in communication, but yellow specifically targets warnings about hazards, making it the appropriate choice for this question.