What instrument is primarily used to determine altitude in an aircraft?

The altimeter is the primary instrument used to determine altitude in an aircraft. It works by measuring the atmospheric pressure and comparing it to a standard atmospheric pressure level, allowing pilots to ascertain their altitude above sea level. The altimeter is crucial for maintaining safe flight operations, especially in mountainous terrain or when flying near other aircraft.

Attitude indicators, while essential for showing the aircraft's orientation in relation to the horizon, do not directly provide altitude information. Similarly, heading indicators are used to determine the aircraft's direction of travel, but they also do not show altitude. The vertical speed indicator indicates whether the aircraft is climbing or descending, but it does not provide the actual altitude measurement itself. Therefore, the altimeter is the only instrument specifically designed for that purpose, making it the correct choice.