What must one pilot do if another pilot leaves the controls above FL350?

When one pilot leaves the controls of an aircraft above FL350, the requirement to use an oxygen mask is based on the physiological effects of altitude on human performance and safety. At altitudes above FL350 (35,000 feet), the partial pressure of oxygen decreases significantly, leading to a greater risk of hypoxia, which can impair a pilot's cognitive and physical abilities.

Using an oxygen mask ensures that the remaining pilot can maintain adequate oxygen levels while managing the aircraft's operation. This is crucial for safety, especially in situations where lack of oxygen could lead to a loss of control or responsiveness. The regulations governing aviation safety necessitate that all pilots use supplemental oxygen above this altitude to mitigate these risks.

Other options do not meet the safety requirements established for operations above FL350. In particular, removing the oxygen mask or using it only in specific conditions could potentially expose the remaining pilot to dangerous levels of hypoxia. Hence, the necessity of using an oxygen mask regardless of type at altitudes above FL350 is a critical safety measure in aviation practices.