Which of the following is NOT one of the four types of chemical agents?

The correct answer is B, Poison gas. In military terminology, chemical agents are classified into specific categories based on their effects and characteristics. The four recognized types of chemical agents are choking agents, blister agents, nerve agents, and incapacitating agents.

Choking agents, such as chlorine and phosgene, primarily harm the respiratory system, causing suffocation or respiratory distress. Blister agents, like mustard gas, cause severe skin and tissue damage upon contact. Nerve agents, such as sarin and VX, disrupt the nervous system, leading to paralysis or death.

While "poison gas" might seem like a term that could refer to any harmful gas used in warfare, it is not an official classification for these agents. The term can be too broad and nonspecific, whereas the other three categories are clearly defined within military doctrine.