Final answer:
A Flight Plan should be filed with an ATC unit, FSS or Community Aerodrome Radio Station, as it is crucial for safety and efficient tracking of the aircraft.
Step-by-step explanation:
The correct answer to the question is D) Flight Plan. A flight plan is a document that provides the important details of a flight to air traffic control (ATC) units, flight service stations (FSS), or community aerodrome radio stations (CARS). It includes information such as the route of the flight, the type of aircraft, and the estimated time of departure and arrival. Filing a flight plan is a critical step for ensuring safety and efficiency in air travel as it allows ATC to track the aircraft throughout its journey and provides necessary information in the event of an emergency.