Which instrument indicates the aircraft's rate of turn?

The turn coordinator is designed specifically to indicate the aircraft's rate of turn. It provides a visual representation of how quickly the aircraft is turning, typically in degrees per second. This instrument includes a colored ball that moves in relation to a curved line, helping pilots achieve standard rate turns (typically 3 degrees per second) by aligning the ball with the line. This allows for better management of turns, ensuring they remain within controlled parameters and are smooth, which is crucial for maintaining situational awareness and flight stability.

The other instruments listed provide different types of information that are not related to the aircraft's rate of turn. The altimeter measures altitude, the airspeed indicator shows the speed of the aircraft relative to the surrounding air, and the heading indicator displays the aircraft's current heading. While the heading indicator can help a pilot understand the direction of the aircraft, it does not provide any information about how quickly the aircraft is changing that heading. Therefore, the turn coordinator is the correct instrument for indicating rate of turn.