What indicates the thermal discharge of a built-in fire extinguisher system?

The correct indication of thermal discharge for a built-in fire extinguisher system is the absence of the red plastic disc in the discharge line. This disc is a critical component of the system; it serves as a seal that maintains the integrity of the fire extinguisher until it is activated. If the disc is missing, it signifies that the extinguisher has been discharged due to a rise in temperature or an activation signal, thus confirming that the system has responded to a potential fire threat.

The other options, while relevant to fire extinguisher systems, do not specifically indicate thermal discharge. A green light on the control panel usually indicates that the system is operational and in a normal state, rather than indicating a discharge. An alarm sound could indicate various issues, including that a fire has been detected or a malfunction in the system, but it doesn't specifically confirm the thermal discharge of the extinguisher. Similarly, a flashing red light typically would signal a warning or alert status, not necessarily that the extinguisher has discharged.