Which instrument can help you determine if you are maintaining a constant attitude?

The altimeter is essential for pilots because it measures altitude above a reference point, usually sea level. When maintaining a constant attitude, it is crucial to ensure that your altitude remains stable, especially during flight maneuvers. By monitoring the altimeter, you can determine if your aircraft is climbing, descending, or maintaining level flight. A constant altitude indicates that the aircraft's attitude is stable, meaning the wings are level and the aircraft is not pitching up or down.

While the other instruments provide valuable information about the aircraft's performance—the airspeed indicator shows how fast the aircraft is moving through the air, the compass indicates direction, and the vertical speed indicator reveals whether you’re climbing or descending at a certain rate—they do not specifically indicate the aircraft's attitude as directly as the altimeter does in the context of maintaining a level flight. Hence, the altimeter is the most effective instrument for determining if you are maintaining a constant attitude.