Final answer:
Air traffic control systems are classified as hard real-time systems because they require strict adherence to timing constraints, where missing a deadline is considered a system failure.
Step-by-step explanation:
Air traffic control systems are critical for ensuring the safety and efficient management of aircraft operations. Such systems require precise timing and coordination, as any delay or failure in processing and reacting to information can lead to catastrophic consequences. Given this, air traffic control systems fit into the category of hard real-time systems. Unlike soft or very soft real-time systems, where the utility of a computation degrades after its deadline, in hard real-time systems, missing a timing constraint is considered to be a system failure. Therefore, the correct answer is D. Hard real-time systems.