When landing on a runway served by a VASI, when is it permissible to descend below the glideslope?

The correct answer is based on safety protocols and the operational use of the Visual Approach Slope Indicator (VASI) system. Descending below the glideslope is permissible when a lower altitude is necessary for a safe landing. This situation may arise due to various factors, such as the aircraft being high on approach, needing to align better with the runway, or additional obstacles present on the approach path.

The VASI provides visual cues to help pilots maintain the correct descent path, but there are circumstances where strict adherence to the glideslope cannot be maintained in favor of safely completing the landing. As long as the pilot can ensure a safe touchdown, deviations below the glideslope can be justified.

In the context of the other choices, clearance from Air Traffic Control (ATC) or timing of descent in relation to runway threshold does not necessarily govern safety on approach—it is ultimately the pilot's responsibility to ensure that the aircraft can land safely. Descending at any time during the approach, on the other hand, could lead to unsafe situations if not managed properly, which is why operational protocols focus on safety as a priority.