Which of the following chemicals is known as a nerve agent?

VX is recognized as a nerve agent due to its mode of action, which interferes with the normal functioning of the nervous system. It is classified as a pesticide but was developed as a chemical weapon. VX inhibits the enzyme acetylcholinesterase, leading to an accumulation of acetylcholine in the body, causing continuous stimulation of muscles, glands, and the central nervous system. This can result in symptoms such as muscle twitching, respiratory failure, and can be fatal if not treated promptly.

The other chemicals listed do not possess the same properties as nerve agents. Mustard gas, for example, is a blistering agent primarily causing damage to the skin and respiratory tract rather than acting directly on the nervous system. Chlorine is a toxic gas that can lead to respiratory issues but does not function as a nerve agent. Ammonia, while hazardous, is primarily an irritant and does not operate through the mechanisms characteristic of nerve agents.