What does an “L” on an IAP frequency signify?

The correct choice is that an “L” on an IAP frequency indicates a localizer approach. In the context of Instrument Approach Procedures (IAPs), the designation of “L” is specifically used to signify that the frequency is associated with a localizer, which plays a crucial role in precision approaches. Localizers provide horizontal guidance for the approach, allowing pilots to align their aircraft with the centerline of the runway.

On the other hand, the option regarding the use for turning on runway lights, while plausible, does not accurately define what the "L" represents in the context of aviation frequencies. The mention of low frequency is misleading, as localizer frequencies generally fall within a specific range that is distinct from other frequency designations. Lastly, while landing notifications are important, they are not indicated by the "L" in the context of IAP frequencies, which is strictly related to localizer approach guidance.