Define "controlled airspace."

The term "controlled airspace" refers to airspace of defined dimensions in which air traffic control (ATC) provides services to aircraft to help ensure safe and orderly operations. This designation allows for the management of air traffic and the separation of aircraft, thereby reducing the risk of midair collisions and facilitating the efficient flow of air traffic within that space.

Controlled airspace is typically classified into several categories (Class A, B, C, D, and E) that vary in terms of the requirements for flight operations, visibility, and pilot qualifications. Within this airspace, pilots are usually required to communicate with air traffic control and adhere to specific regulations.

The other choices suggest scenarios that do not accurately define controlled airspace. For example, airspace without restrictions does not require ATC oversight, and airspace solely for military aircraft does not reflect the broader protections that ATC provides to all types of aircraft. Similarly, the notion of controlled airspace being free of traffic does not hold true, as controlled airspace actively manages traffic to ensure safety. Thus, defining controlled airspace as that which is subject to air traffic control captures its essential operational characteristics.