Which of the following is a supporting instrument for maintaining heading in straight-and-level flight?

The turn coordinator is a supporting instrument for maintaining heading in straight-and-level flight because it indicates the rate of turn and helps the pilot recognize whether the airplane is in a coordinated turn or is in straight flight. While flying straight and level, the airplane should maintain a constant heading, and the turn coordinator provides visual cues when the aircraft is turning, thus allowing the pilot to remain aware of any deviation from the intended heading.

In this instrument, a properly maintained level flight should show no movement on the turn coordinator, indicating that the aircraft is neither turning nor rolling. This makes it an essential tool for ensuring that the aircraft's heading remains constant during flight.

The other instruments listed serve different primary functions related to flight. The altimeter measures altitude, the vertical speed indicator (VSI) displays the rate of ascent or descent, and the airspeed indicator shows the speed of the aircraft relative to the surrounding air. While these instruments are crucial for overall flight management, they do not specifically aid in maintaining a heading like the turn coordinator does.