What type of control tower is referenced in the definition of Class D airspace?

Class D airspace is defined as airspace that has an operating control tower. This means that air traffic control services are available to aircraft operating within that airspace, providing essential guidance and coordination to ensure safety during takeoff, landing, and while in the vicinity of the airport.

An operable control tower signifies that air traffic controllers are present and able to provide services, which is a key characteristic of Class D airspace. The reference to Class D in the context of airspace assumes the active role of the tower in managing the air traffic. In contrast, choices like inoperative, temporary, and permanent do not correctly reflect the status of the control tower as required for Class D airspace, as they either imply a lack of functionality or do not pertain to the consistent operational nature of the airspace classification.