Which of the following chemical agents is classified primarily as a blister agent?

Mustard gas, known chemically as sulfur mustard and represented by symbols like H, HD, or HN, is classified primarily as a blister agent due to its severe effects on human skin and internal organs. When it comes into contact with skin or is inhaled, mustard gas causes painful blisters and damage to the respiratory tract, making it a potent chemical warfare agent. Its use has been historically documented in conflicts, where it was notorious for causing prolonged suffering due to its delayed effects and painful injuries.

The other agents listed do not primarily function as blister agents. Soman, for instance, is classified as a nerve agent, which affects the nervous system rather than causing blistering. Phosgene oxime is also regarded as a vesicant but is less recognized compared to mustard gas, which is why it does not take precedence in classification. Methylphosphonate does not correspond to a blistering agent's characteristics but is related to nerve agents. Therefore, the classification of mustard gas as a blister agent is well-founded based on its severe dermatological and respiratory impacts.