What sign is used to indicate a radioactive material?

The yellow and trefoil symbol is indeed the correct indication for radioactive materials. This iconic symbol, which resembles a three-bladed propeller or trefoil, is specifically designed for the identification of radioactive substances and materials. Its distinctive shape and bright yellow color help to ensure visibility and recognition, which are crucial in safety and hazard communication. The trefoil symbol emphasizes the potential danger associated with radiation, making it well understood in various settings, including transportation, storage, and emergency response situations.

The other symbols listed do not correspond to radioactive materials and serve different purposes or signify different hazards. For example, the blue and square symbol may represent various safety or informational warnings, while the red triangle is more commonly associated with flammable materials or conditions. The green circle typically indicates general safety information or favorable conditions rather than any specific hazard like radioactivity. Therefore, the yellow and trefoil symbol is uniquely relevant and universally accepted for identifying radioactive materials.