Which of the following is a signal that indicates a fire alarm system issue?

A supervisory signal is an important indication within a fire alarm system that points to an issue affecting the system's performance or integrity. It is not a direct alarm indicating a fire, but rather alerts personnel to problems such as an issue with the system's components, a potential malfunction, or system supervision anomalies.

For example, if a fire alarm panel detects a trouble condition, it may generate a supervisory signal to inform operators that maintenance or inspection is required. This can include conditions such as a low battery, loss of AC power, or a fault in the wiring. Such signals are critical for ensuring that the fire alarm system is functioning correctly and ready to respond in the event of an actual fire.

In contrast, a green light indicator typically signals that the system is operating normally. A test signal is used for ensuring that components function correctly during routine checks, and manual alarm activation initiates an emergency response; neither of these actions indicate a malfunction or problem within the system. Hence, the supervisory signal is the clear indicator of an issue within the fire alarm system.