Which components are part of the ILS?

The correct choice identifies the components integral to an Instrument Landing System (ILS), which are critical for precision approaches to runways. The glideslope provides the vertical guidance necessary for aircraft to descend at the correct angle during the approach. The localizer offers horizontal guidance by indicating the aircraft's position relative to the runway centerline, ensuring that the aircraft is aligned properly with the runway. Marker beacons serve as additional navigational aids, notifying pilots of their distance from the runway threshold during the approach. Finally, approach lights enhance the visual reference for pilots, particularly in low visibility conditions, aiding them in transitioning from instrument flying to visual landing.

The other options contain components that do not belong to the ILS specifically. For example, while a beacon provides navigational assistance, it is not part of the ILS. Similarly, radar is typically used for air traffic control and not for guiding approaches in the ILS framework. The other options list systems such as VOR, DME, and GPS, which are related to navigation but do not assemble the components that form an ILS approach system.