What color and symbol combination is used on a placard to indicate a radioactive hazard?

The correct answer is yellow and white with a propeller, which represents a radioactive hazard. This combination is consistent with the labeling and placarding systems used for hazardous materials, where specific colors and symbols are designated for different types of hazards.

The yellow background serves to catch the attention of anyone who may encounter the material, indicating caution is required. The propeller symbol is an internationally recognized representation of radioactive materials, alerting responders and the public to the specific danger associated with radiation.

Other symbols and colors are associated with different hazards, which helps distinguish between various types of potential threats in hazardous materials situations. For example, red and white with a flame is generally used for flammable materials, while blue and white with a water droplet represents materials that are water-reactive, further emphasizing the importance of proper labeling for safety and emergency response.