Which component is essential for accurately determining an aircraft's heading using magnetic systems?

The magnetic compass is essential for accurately determining an aircraft's heading using magnetic systems because it directly responds to the Earth's magnetic field. The compass aligns itself with magnetic north, allowing pilots to read their heading relative to this reference. This is particularly important in navigation, as pilots need a reliable indication of their directional orientation during flight.

Other components listed, such as the true airspeed indicator, flight data recorder, and ground proximity warning system, provide valuable information to pilots, but they do not contribute directly to determining the aircraft's magnetic heading. The true airspeed indicator measures the speed of the aircraft through the air, while the flight data recorder logs various flight parameters for later analysis. The ground proximity warning system alerts pilots to terrain and obstacle proximity, enhancing safety, but it does not yield information related to heading. Thus, the magnetic compass is the key instrument in this context for establishing an aircraft's heading using magnetic systems.