How does the heading indicator function?

The correct choice is that the gyro spins in a horizontal plane. The heading indicator, commonly known as the directional gyro, operates using a gyroscope that maintains its orientation while the aircraft maneuvers. When the gyro spins in a horizontal plane, it helps the instrument accurately reflect the aircraft's heading relative to a specific reference point, typically true north.

This horizontal spinning character of the gyroscope allows it to remain stable in its orientation, which is vital for providing correct heading information. The inertia of the spinning gyroscope resists changes in orientation, making the heading indicator accurate and reliable as long as there are no significant errors caused by precession or other factors.

While other choices reference aspects of aircraft movement and other systems like GPS, they do not accurately describe the fundamental operational principle of how a heading indicator functions in flight. The precision of the gyro spinning in a horizontal plane directly contributes to the instrument's key role in navigation and orientation.