Which fire is specifically linked to unauthorized activities?

The correct answer is "Illegal fire" because this term specifically denotes fire incidents that are associated with unauthorized or unlawful activities. It encompasses fires that may be intentionally set without the proper permits or for illicit purposes, such as vandalism or arson. This classification highlights the deliberate nature of the act, often resulting in legal consequences.

Car fires, trash fires, and fire alarms represent different contexts. Car fires may occur due to accidents, mechanical failures, or even arson, but they are not inherently linked to unauthorized activities unless specified as such. Trash fires can also happen due to negligence or accidental causes rather than being specifically linked to illegal actions. Fire alarms serve as a safety measure rather than a type of fire, being directly related to the detection of smoke or fire rather than the causes behind these fire incidents. Thus, "Illegal fire" is the most accurate option as it directly references the unlawful nature of the act.