What are threshold lights used for in aviation?

Threshold lights are critical visual aids in aviation, particularly during the landing phase of an aircraft's approach. They are strategically positioned at the beginning of the runway to define the landing threshold, which is the point where a runway begins. This marking is essential for pilots to accurately identify the runway's starting point, ensuring safe landings and effective runway utilization.

When approaching for a landing, visibility of the threshold lights provides pilots with a clear indication of the landing zone on the runway, which is crucial, especially in low visibility conditions. The presence of these lights helps maintain situational awareness and contributes to the overall safety and efficiency of flight operations.

In contrast, the other options focus on different aspects of airport operations, such as marking taxiways, indicating runway length, or providing takeoff guidance. While these elements are important in their own right, they do not specifically relate to the primary function of threshold lights in aviation.