In which class of airspace are both IFR and VFR permissible without ATC clearance?

Class D airspace is the correct answer because it allows both Instrument Flight Rules (IFR) and Visual Flight Rules (VFR) operations without the need for an Air Traffic Control (ATC) clearance, provided that the pilot can maintain adequate separation from other aircraft. In Class D airspace, ATC provides services to both IFR and VFR flights, but VFR pilots are not required to communicate with ATC unless they choose to.

This class of airspace typically surrounds airports with control towers and may have specific operational requirements, but fundamentally it does not require ATC clearance for entry or operation under VFR. Other classes, such as Class B and Class C, have stricter requirements and necessitate ATC clearance for all aircraft, while Class F airspace can be a non-controlled airspace type that may also have its own specific regulations but is not widely used in the United States airspace system.