What may trigger an unnecessary alarm?

An unnecessary alarm can be triggered by various common sources that may not indicate a true fire hazard. Among these, a lit cigarette in a non-smoking area is a situation that could lead to a smoke detector activating even though there is no immediate danger. Smoke detectors are designed to be sensitive to even small particles in the air, and cigarette smoke can certainly cause this reaction.

The other choices also comprise valid scenarios that can lead to alarm activation. Smoke from a cooking appliance is a frequent cause of false alarms in residential settings, especially if food is burnt. Additionally, a defective alarm system could result in false alarms due to malfunctions.

While choice B identifies a specific scenario that can trigger an unnecessary alarm, the comprehensive option indicating "all of the above" encompasses the range of typical situations that can lead to such an event. Understanding these triggers is crucial for effectively managing smoke detectors and minimizing false alarms.