What instrument's primary function is to help maintain straight and level flight?

The primary function of the artificial horizon, or attitude indicator, is to show the aircraft's orientation relative to the horizon. This instrument displays whether the aircraft is climbing, descending, or in level flight and indicates the bank angle. By providing this crucial information, it assists pilots in maintaining straight and level flight, particularly in conditions where the natural horizon is not visible, such as in clouds or at night. The attitude indicator helps pilots make the necessary adjustments to their pitch and roll to maintain the desired flight path.

The role of the other instruments, while important for overall situational awareness and safety, does not focus primarily on maintaining straight and level flight. The airspeed indicator helps monitor the aircraft's speed relative to its stall speed and optimal performance, the heading indicator reflects changes in the aircraft’s direction, and the vertical speed indicator indicates the rate of climb or descent, each serving different but essential purposes in flight management.