Which lights serve to identify the ends of the runway?

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Runway end identifier lights are specifically designed to indicate the ends of the runway to pilots, particularly during approach and landing. These lights typically consist of a pair of lights located at the runway's threshold—one on each side—and they flash in a manner that provides clear visibility of the runway's endpoint, especially in low-visibility conditions or at night.

The other options, while associated with runway operations, serve different purposes. For instance, runway edge lights are used to define the usable edges of the runway, providing guidance but not specifically marking the runway ends. Threshold lights indicate the beginning of the runway's usable area but do not serve as identifiers for the very end. PAPI lights are used for approach slope guidance to help pilots maintain the correct altitude during landing, again not specifically marking the runway ends. Therefore, the runway end identifier lights are the correct choice for identifying the ends of the runway.