What is the primary purpose of Air Traffic Control?

The primary purpose of Air Traffic Control (ATC) is to ensure the safe and efficient movement of aircraft in the airspace system. This encompasses preventing collisions not only between aircraft but also between aircraft and physical obstacles on the ground or in the air, which is critical for maintaining safety in increasingly busy airspace. Furthermore, ATC plays a vital role in managing and expediting an orderly flow of air traffic, which is important for minimizing delays and improving the efficiency of air travel.

The other options, while they touch on aspects related to air traffic management, do not fully capture the comprehensive role of ATC. For instance, preventing air traffic from entering congested airspace is part of ATC's function but is more of a subset of the broader goal of collision avoidance and traffic flow management. Checking aircraft security is essential but falls under a different domain, typically associated with airport security measures rather than ATC operations. Lastly, controlling air traffic for military needs can occur, but the overarching aim of ATC is broader than just military purposes, focusing primarily on civil air traffic and safety for all aircraft.