Federal Emergency Management Agency (FEMA) Standardized Awareness Practice Test

The answer provided is the most appropriate choice as tear gas is a well-known riot control agent used by law enforcement to disperse crowds. It is specifically designed to irritate the eyes, respiratory system, and skin, making it an effective tool for crowd control situations. Tear gas can cause tearing, coughing, and temporary blindness, which helps to manage and disperse hostile groups without permanent injury when used correctly.

In contrast, while chlorine is a hazardous substance often associated with toxic gas incidents, it is not primarily used for crowd control. Hydrogen cyanide is a highly toxic compound and while it can incapacitate individuals, it is not categorized as a riot control agent. Phosgene, another toxic chemical, is primarily known for its use in chemical warfare and has severe respiratory effects, but it is not appropriate for riot control purposes.

Thus, tear gas stands out as the most recognized and widely used chemical specifically designated for controlling riots and managing civil disturbances.