What happens to Class D airspace when the control tower is not in operation?

When the control tower at an airport classified as Class D airspace is not in operation, that airspace automatically transitions to Class E airspace. This is because Class D airspace is dependent on the active operation of an air traffic control tower. When the tower shuts down, the controlled nature of Class D airspace is no longer valid, leading to its classification changing to Class E.

Class E airspace is designated as controlled airspace but does not require air traffic control services to be provided. In this scenario, pilots still must adhere to general aviation rules and regulations, but they are no longer under direct control of an air traffic controller. The other options do not accurately represent what occurs in this situation, as Class C airspace requires a different set of operational conditions, while uncontrolled airspace refers to areas where air traffic control does not provide services.