What do threshold markings indicate at the end of runways?

Threshold markings are specific visual cues located at the ends of runways that provide important information to pilots. The correct answer describes these markings as 12 feet wide and 150 feet long parallel stripes. This standardized design helps pilots easily identify where the runway begins, which is crucial during the landing phase.

The markings serve to delineate the usable portion of the runway, clearly indicating the threshold from which aircraft can safely commence their landing procedures. The dimensions and characteristics of the markings are established by regulatory bodies to ensure uniformity and clarity across different airports, thereby aiding in the safe operation of aircraft.

In contrast, while the other options mention critical aspects related to runway operation, they do not accurately describe what threshold markings specifically indicate. For instance, the end of the takeoff area may refer to a separate aspect of runway usage, while the cleared landing zone and obstacle-free zone pertain to areas adjacent to the runway rather than the markings themselves. Understanding the definition and purpose of runway threshold markings is essential for navigation and safety in aviation.