What is the significance of the "threshold" on a runway?

The threshold of a runway is a critical component of airport operations as it marks the beginning of the usable portion of the runway for landing aircraft. This is significant because it informs pilots where they can safely initiate their landing approach, ensuring that they touch down in an appropriate area that can accommodate their aircraft type and landing procedure. The threshold is typically identified by specific markings on the runway that assist pilots in visualizing their landing point.

Understanding the location of the threshold is essential for both safety and operational efficiency. For example, it helps in determining the aircraft's approach angle, descent rate, and landing speed. A correct identification of the threshold ensures that landings are conducted in a controlled and safe manner, which is crucial for preventing overrun incidents and ensuring that the aircraft has adequate distance to come to a stop.

Other choices, while related to runway operations, do not capture the specific role of the threshold. For instance, markings for airport boundary lines or visual references for diversions serve different purposes and are not directly about the usable landing area. Elevation points are important in overall runway management but do not define the start of usable runway space for landings. Therefore, option C accurately reflects the essential function of the runway threshold in aviation safety and operations.