Which instrument is crucial for providing an immediate indication of an airplane's pitch and bank attitude?

The attitude indicator is essential for providing an immediate and clear representation of an airplane's pitch and bank attitude. This instrument displays the orientation of the aircraft relative to the horizon, allowing pilots to see whether the wings are level and if the nose is above or below the horizon line. This information is crucial for maintaining control of the aircraft during various phases of flight, especially in conditions where visual references outside the cockpit may be limited, such as in clouds or at night.

In contrast, the vertical speed indicator measures the rate of climb or descent, but it does not provide any information about the aircraft's pitch or bank angle. The altimeter indicates the aircraft’s altitude above sea level, while the heading indicator provides information on the direction the aircraft is facing. Neither of these instruments provides the direct feedback about the aircraft's attitude necessary for effective flight management, making the attitude indicator the key instrument in assessing pitch and bank.