Which of the following performance instruments is used to indicate altitude?

The altimeter is specifically designed to indicate altitude within an aircraft. It measures the atmospheric pressure outside the aircraft and translates that pressure reading into an altitude above sea level. This is crucial for pilots, as it allows them to maintain the correct altitude during flight, avoiding potential obstacles and ensuring safe navigation within controlled airspace.

In contrast, while the Mach Speed Indicator (MSI) measures the aircraft's speed relative to the speed of sound, and the Rate of Climb/Vertical Speed Indicator (RA/VSI) indicates how quickly the aircraft is climbing or descending, neither provides a direct measurement of the absolute altitude. The Heading Indicator assists pilots with directional navigation but does not furnish altitude information. Thus, the altimeter stands out as the essential instrument for altitude indication in aviation.