What must occur for the PMU to go offline?

The correct answer is based on the operational safety protocols for the PMU (Power Management Unit) in the context of system temperature thresholds. When the inlet oil air temperature (IOAT) exceeds 121 °C, it indicates a critical overheating condition. At this temperature, the PMU is designed to go offline to prevent damage to the system and to ensure operational safety. High temperatures can lead to failure of electronic components and could compromise the integrity of the system.

Maintaining the temperature within safe limits is crucial for system reliability and longevity, which is why strict thresholds like the one at 121 °C are set. Going offline at this point allows for necessary cooling and prevents potential hazards that could arise from prolonged exposure to excessive heat. This decision is part of the preventative measures taken to protect not only the PMU but the entire system from heat-related failures.

The other temperature options, while they may indicate elevated temperatures, do not trigger the offline condition. Thus, they do not meet the critical threshold required for the PMU's protective measures to activate.