Which agent is primarily used in chemical warfare?

The primary agent used in chemical warfare among the given options is the Vx nerve agent. Vx is an extremely potent nerve agent that disrupts the normal functioning of the nervous system by inhibiting the enzyme acetylcholinesterase, leading to an accumulation of acetylcholine. This accumulation causes continuous stimulation of muscles, glands, and the central nervous system, which can ultimately result in respiratory failure and death if not treated promptly.

Vx is classified as a chemical weapon and has been specifically developed for military use, highlighting its significance in warfare scenarios. The effects of Vx are rapid and can be fatal, making it one of the most dangerous chemical agents.

While other listed agents like chlorine gas and mustard gas have been used historically in warfare and can result in significant harm, they do not match the strategic military application and lethality of Vx nerve agent. Botulinum toxin, though extremely toxic, is mainly associated with foodborne illness and is not generally classified as a chemical warfare agent despite its potency.