What action must a flight crewmember take if the other pilot leaves the flight deck at FL390?

When one pilot leaves the flight deck at high altitude, such as FL390, it is crucial for the remaining pilot to prioritize safety and ensure that they can effectively manage the flight alone. The use of an oxygen mask becomes essential in this scenario because cabin pressure may be compromised if there is a sudden decompression, and the remaining pilot must maintain consciousness and cognitive function to operate the aircraft safely. The supplemental oxygen provided by the mask ensures that the pilot can continue to breathe normally and manage any potential emergencies that may arise from the absence of the other pilot. This action is a standard safety protocol that emphasizes the flight crew's responsibility to safeguard the aircraft and its occupants under varying conditions.

While notifying air traffic control, preparing for descent, or contacting ground services may be relevant actions in different contexts, they do not directly address the immediate physiological need and the safety requirements that arise when a pilot vacates the flight deck at such high altitudes. The priority in this situation is securing adequate oxygen supply, making the use of an oxygen mask the correct choice.