What instrument provides information about the aircraft's bank angle?

The attitude indicator is specifically designed to provide pilots with information about the aircraft's orientation relative to the horizon, which includes the bank angle. This instrument displays the aircraft's pitch and roll in relation to a reference horizon, allowing pilots to see how much the aircraft is banked during flight. It uses a gyroscope to indicate whether the wings are level or if the aircraft is in a turn, which directly correlates to the bank angle.

While other instruments like the turn coordinator do provide information related to turns and can indicate the rate of turn, they do not directly represent the bank angle in the same intuitive manner as the attitude indicator. The directional gyro and horizontal situation indicator focus on heading and situational awareness, respectively, rather than directly indicating bank angle. Therefore, the attitude indicator is the correct choice for determining an aircraft's bank angle.