Which of the following is an example of a chemical agent used in warfare?

The correct answer is that all options presented are examples of chemical agents used in warfare. Mustard gas, phosgene, and sarin gas have historically been employed in military conflicts, demonstrating their categorization as chemical agents.

Mustard gas, a blistering agent, was extensively used during World War I, causing severe injuries and long-term health effects. Phosgene, a potent respiratory agent, can lead to pulmonary edema and was also used historically in WWI. Sarin gas, a nerve agent, can disrupt the nervous system, leading to respiratory failure and death, making it one of the most lethal chemical agents used.

Thus, each item listed represents a significant type of chemical weapon, justifying the conclusion that all of the choices are correct examples of chemical agents utilized in warfare.