Which of the following chemical agents has been widely used as a riot control agent?

The choice of tear gas as a widely used riot control agent is correct due to its effectiveness in incapacitating individuals temporarily by causing tearing, pain, and blurred vision. Tear gas typically contains chemicals like CS gas or pepper spray, which serve to disperse crowds in control situations. Its formulation is intended to create discomfort, prompting individuals to move away from the area, thus helping to restore order without causing permanent harm.

In contrast, choking agents are designed to affect the respiratory system and can cause long-term health issues if exposed to them. Vomiting agents, intended to induce nausea and vomiting, are less effective for crowd control compared to tear gas. Blister agents primarily cause severe skin and respiratory damage and are not suitable in non-lethal crowd control scenarios. Hence, the unique properties of tear gas make it the preferred choice among law enforcement for managing riots or disturbances.