Under what circumstances will oxygen masks automatically deploy?

Oxygen masks automatically deploy during an event of cabin decompression because this situation poses a significant risk to passengers and crew due to the lack of adequate oxygen at higher altitudes. When cabin pressure drops suddenly, it can lead to a condition known as hypoxia, where the body doesn't receive enough oxygen, impairing bodily functions and causing confusion or unconsciousness.

By deploying oxygen masks automatically in the case of decompression, the aircraft system ensures that passengers have immediate access to supplemental oxygen, allowing them to breathe normally until the issue can be resolved or the aircraft descends to a safer altitude where sufficient cabin pressure can be restored. This safety feature is crucial for protecting the health and safety of everyone on board during an emergency situation.

In contrast, the other options do not reflect scenarios that require immediate deployment of oxygen masks. An aircraft landing, routine maintenance checks, and the seatbelt sign being off do not inherently compromise cabin pressure or the availability of breathable air, thereby not necessitating the use of oxygen masks.