Which type of light is primarily used to outline the edges of a runway?

The option that represents the correct answer describes the specific function and location of runway edge lights. These lights are designed to illuminate the edges of the runway during periods of low visibility, such as at night or during poor weather conditions. By outlining the runway's perimeter, they provide critical visual guidance for pilots approaching and departing from the runway, helping to ensure safe navigation.

The other options represent different lighting systems with distinct purposes. Threshold lights indicate the beginning of the usable runway, marking the point where the runway starts for landing or takeoff. The approach lighting system consists of a series of lights that help pilots transition from instrument flight to visual flight as they approach the runway. Landing lights, typically located on the aircraft itself, are used to illuminate the runway and surrounding area during landing, but they do not outline the runway edges.