Which instrument is primary for maintaining proper pitch during straight-and-level flight?

The primary instrument for maintaining proper pitch during straight-and-level flight is the attitude indicator. This instrument provides pilots with visual information about the aircraft's orientation relative to the horizon. It shows whether the aircraft is climbing, descending, or maintaining level flight.

In straight-and-level flight, the attitude indicator displays a level flight position, indicating that the wings of the aircraft are level with the horizon. By monitoring this instrument, a pilot can make necessary adjustments to the elevator control to maintain the desired pitch and, therefore, the altitude.

The other instruments listed serve different functions: the vertical speed indicator (VSI) measures the rate of climb or descent, the altimeter provides information about altitude but does not directly indicate pitch, and the airspeed indicator reflects the aircraft's speed through the air. While all these instruments are important for overall flight management, the attitude indicator is specifically critical for maintaining proper pitch during straight-and-level flight.