What controls the deployment of passenger oxygen masks?

The deployment of passenger oxygen masks is primarily controlled by cabin pressure altitude. When there is a significant change in cabin pressure, usually due to an emergency situation such as rapid decompression, the oxygen masks are designed to automatically deploy. This system is calibrated to recognize when the cabin altitude exceeds a certain threshold, typically around 14,000 feet, which is when supplemental oxygen becomes necessary for passenger safety.

The other options, although they may play roles in certain scenarios, do not directly determine the automatic deployment of the masks. The emergency switch is typically used to activate specific safety protocols manually and may not be the primary control for passenger mask deployment. Flight crew discretion is certainly important in managing emergencies, but the masks should deploy automatically based on pressure changes without requiring crew intervention. A manual trigger near the exit could be part of the safety mechanisms but does not supersede the automatic deployment system that operates based on cabin pressure. Thus, the mechanism governing the release of the oxygen masks is firmly tied to changes in cabin pressure altitude, making it the correct answer.