Which instrument indicates the aircraft's bank angle?

The instrument that indicates the aircraft's bank angle is the artificial horizon, also known as an attitude indicator. This instrument displays the aircraft's orientation in relation to the horizon, showing whether the wings are level, or if the aircraft is climbing, descending, or turning. The artificial horizon is essential for pilots, especially when flying in conditions where visual reference to the outside environment is limited, such as in clouds or at night. It helps maintain controlled flight by providing an immediate visual representation of the aircraft's bank angle and pitch.

In contrast, the altimeter measures altitude by detecting changes in air pressure, the vertical speed indicator shows the rate of climb or descent in feet per minute, and the airspeed indicator displays the aircraft's speed relative to the surrounding air. While all of these instruments are critical for flying, they do not indicate bank angle, which is the specific function of the artificial horizon.