What does an "L" enclosed in a circle on En Route Low Altitude Charts represent?

An "L" enclosed in a circle on En Route Low Altitude Charts signifies "pilot controlled lighting." This symbol indicates that pilots can activate lighting systems at airports or on runways that do not have continuous lighting. The ability to control the lighting via the aircraft's radio allows pilots to enhance visibility for landing and takeoff, especially during night operations or low-visibility conditions. This feature is particularly useful at smaller airports, where such lighting may not be continuously operational, thus ensuring that pilots have the necessary visibility to navigate safely.

The other options pertain to different functions within the air traffic control and navigation framework but do not specifically relate to the symbol denoting pilot controlled lighting.