Which symbol serves as a warning for chemical hazards involving riot control agents and smoke?

The correct choice is yellow, which is the designated symbol for indicating chemical hazards related to riot control agents and smoke. Yellow is universally recognized in various safety and hazard communication systems to signify caution and the presence of potentially harmful substances. When dealing with riot control agents, which can include chemical irritants, it is crucial to understand the risks associated with these materials. The yellow color helps personnel quickly identify and acknowledge the specific hazards posed, ensuring that the appropriate safety measures and protective gear are utilized.

In contrast, the colors associated with other hazards do not convey the same meaning related to chemical warnings. For example, red is generally used to indicate fire hazards or emergency situations, blue is often associated with information or mandatory actions, and green typically signifies safety or the absence of hazards. Therefore, using yellow for chemical hazards effectively communicates the necessary caution that handlers of riot control agents need to observe.