Final answer:
Pressing the MASK MAN ON pushbutton deploys oxygen masks for passengers on an aircraft during emergencies like cabin depressurization. It ensures passengers have enough oxygen until normal conditions are restored.
Step-by-step explanation:
When the MASK MAN ON guarded pushbutton is pressed on an aircraft's oxygen panel, option A is the correct answer: Oxygen masks deploy for passengers. This is an emergency measure typically used when the cabin pressure drops unexpectedly, and it helps to ensure that passengers have enough oxygen until the situation is rectified. In situations of sudden cabin depressurization at high altitudes, oxygen masks will automatically drop down. Passengers are instructed to put the masks on immediately to receive a steady supply of oxygen. The physiological adjustment to altitude changes occurs through variations in DPG levels which affect the oxygenation of hemoglobin. Similarly, in hyperbaric medicine, such as when divers undergo recompression or hyperbaric oxygen therapy, supplemental oxygen is provided to ensure safe and adequate oxygen levels at the adjusted pressures.