What type of distance does a DME display measure?

The correct answer is the measurement of slant range in nautical miles (NM) provided by a Distance Measuring Equipment (DME) display. DME operates by calculating the time it takes for a radio signal to travel from the aircraft to the DME ground station and back. This time is then converted into a distance, which is referred to as slant range.

Unlike horizontal distance, which would only consider the distance straight across to the DME without accounting for altitude, slant range takes into consideration the vertical component as well. Therefore, it represents the actual line-of-sight distance from the aircraft to the DME station, regardless of the aircraft's altitude relative to the station.

Understanding this concept is crucial for pilots, as slant range can differ significantly from horizontal distance, particularly when operating at high altitudes or when the DME is located at a different elevation than the aircraft.

The other options, such as altitude in feet, horizontal distance in miles, and ground distance in kilometers, do not accurately describe what DME measures, as they don't reflect the function and operation of the DME system specifically designed to provide slant range distance.