What is the primary function of a 'Flight Plan' in ATC?

Answer: C — document detailing a pilot's intended route and altitudes

The primary function of a 'Flight Plan' in Air Traffic Control (ATC) is to serve as a comprehensive document that details a pilot's intended route, altitudes, and other crucial information regarding the flight. This document is essential for ensuring the safe and efficient management of air traffic. It informs air traffic controllers of the planned path an aircraft will take, including waypoints, departure and arrival airports, expected altitudes, and estimated times. This information allows ATC to track flights accurately, coordinate traffic, provide necessary clearances, and ensure that all aircraft operate safely within their assigned airspace. The flight plan also plays a critical role in search and rescue operations, as it provides a baseline for understanding where an aircraft is expected to be at any given time. In cases where a flight deviates from its filed plan, controllers can react accordingly, enhancing safety in the skies.