What defines "controlled airspace" in aviation?

Controlled airspace is primarily defined by the presence of Air Traffic Control (ATC) services that provide separation and control of aircraft operating within that airspace. This type of airspace is designated to ensure the safety and management of air traffic, where ATC actively monitors and organizes the flow of aircraft, helping to prevent collisions and facilitate efficient operations.

In controlled airspace, communication between pilots and ATC is not just encouraged but is mandatory for safe and effective aircraft operations. This ensures that all participating aircraft operate within known parameters and under established guidelines, enhancing overall aviation safety. Flight activities such as approach, departure, and en-route operations occur in this environment under ATC's guidance, which is crucial in managing busy air traffic hubs.

The other options do not accurately represent the essence of controlled airspace. Communicating with ATC is a requirement in controlled airspace, and while there are altitude restrictions, these alone do not define the airspace as "controlled." Additionally, controlled airspace does not imply that it is free from flight activity; on the contrary, it typically has numerous aircraft operating within it at any given time.