Which function does the localizer serve in the ILS?

The localizer is a key component of the Instrument Landing System (ILS) that provides horizontal guidance to an aircraft during its approach to landing. By emitting radio signals that define the centerline of the runway, the localizer helps pilots maintain the correct lateral alignment with the runway. When a pilot is on approach, the localizer information is critical for ensuring that the aircraft stays on the correct track and aligns properly as it descends towards the runway threshold.

This function is distinct from the other choices, which refer to different aspects of approach and landing; for example, defining the approach angle relates more to the glide slope, which provides vertical guidance rather than horizontal. The localizer does not inform pilots of elevation, as that is not within its capabilities, nor does it signal approach landing speed, which is determined by other flight parameters and not provided through the localizer signal. Thus, the primary role of the localizer is indeed to guide the pilot horizontally during the landing approach.