Which of these is a supplementary component of an ILS?

The correct answer is that compass locators are indeed a supplementary component of an Instrument Landing System (ILS). An ILS comprises localizers, glide slope components, and markers that guide aircraft during the final approach to runways. Compass locators enhance the ILS by providing navigational assistance to pilots, especially in locations where traditional navigational aids might fall short. They operate on low-frequency radio waves and help approximate the aircraft's position relative to the runway during the approach.

While standard terminal arrival routes, visual approach slope indicators, and aerodrome lighted beacons are all valuable components of airport operations and navigational guidance, they do not specifically serve as supplementary elements to the ILS itself. Standard terminal arrival routes are designed to facilitate the arrival of aircraft into terminal areas but are not integrated into the precise approach functions of the ILS. Visual approach slope indicators provide a visual cue for the correct approach angle but do not aid in the electronic guidance offered by an ILS. Aerodrome lighted beacons are typically used for situational awareness and identification of the airport, rather than as a component of the ILS framework.