Which of the following entities is NOT classified as a peace officer?

In the context of the law, a peace officer is generally defined as a person who has been granted the authority to maintain public peace and enforce the law. This role typically includes responsibilities such as making arrests, conducting investigations, and carrying out various duties in relation to the enforcement of laws.

A state trooper, border patrol agent, and local sheriff are all recognized as peace officers because they have specific law enforcement powers and duties aligned with maintaining public order and safety. They are authorized by the state or federal government to perform law enforcement functions and are often involved directly in crime prevention, investigation, and arresting individuals who violate the law.

On the other hand, a city firefighter's primary duties revolve around fire prevention, firefighting, and emergency response related to fire hazards. While firefighters do sometimes receive training in emergency medical response and may assist in certain law enforcement situations, they do not have the inherent authority to enforce laws or make arrests like peace officers do. Therefore, a city firefighter is not classified as a peace officer, which makes this the correct choice in the given question.