Which is one of the three cardinal rules for air traffic control?

One of the three cardinal rules for air traffic control is that landing aircraft should clear the active runway before the next aircraft begins to taxi. This rule is vital for maintaining safety and order within airspace and on runways. It ensures that there is no overlap or potential for collision between aircraft that are landing and those that are preparing to take off or taxi. By requiring that landing aircraft clear the runway first, air traffic control can manage the flow of air traffic more effectively and minimize the risk of accidents. This operational standard contributes to the overall safety and efficiency of aviation operations.

The other options do not reflect accepted safety protocols. For instance, simultaneous takeoffs can introduce significant safety risks, especially at busy airports. Encouraging unguided landings contradicts the need for structured air traffic management, and allowing sling loads to fly over assembled personnel raises serious safety concerns that can lead to hazards for both the personnel and the aircraft involved.