What height must be added for a visual approach light system to descend below during an approach?

The correct response indicates that a visual approach light system should be descended below when the aircraft is 100 feet above the touchdown zone. This standard ensures that the pilot has visual reference to the lights, which are designed to guide the aircraft during the final stages of approach and landing. By setting the descent limit at 100 feet above the touchdown zone, the procedure allows pilots to safely align with the runway while being visually aided by the lights, improving the landing process.

Descent thresholds are critical in aviation because they help enhance situational awareness and landing safety, especially in low visibility conditions. The visual approach lighting serves to alert pilots about their final position and ensure they are on a correct glide path to the runway.

In contrast, other answer options suggest descent altitudes that do not align with aviation guidelines for visual approach systems, which specifically focus on maintaining visual contact with the approach lights at or just before touchdown.