Which condition requires immediate descent to prevent further cabin pressure loss?

Immediate descent is necessary when pressure relief valves are open and the cabin pressure is decreasing. This scenario indicates that the cabin is experiencing a loss of pressure beyond normal operational limits, which could lead to hypoxia for the crew and passengers if not addressed promptly.

When pressure relief valves are open, they allow cabin air to escape if the pressure is exceeding safe operating levels. However, if the pressure is observed to be decreasing, it signifies that the cabin is not maintaining a safe environment. In such circumstances, the optimal action is to initiate a descent to a lower altitude where the atmospheric pressure is higher, ensuring the safe operation of the aircraft and safeguarding the well-being of everyone onboard.

In assessing the other choices, while having all pressure relief valves closed might pose issues, it doesn't directly lead to a loss in cabin pressure on its own. An open and stable pressure condition indicates that the cabin is functioning properly and does not warrant an immediate descent. Similarly, malfunctioning pressure relief valves may pose risks but do not inherently require an immediate response unless they correlate with the cabin pressure decreasing, which would render that option relevant.