Which terrain flight mode is defined by being as close to the surface as possible and characterized by varying airspeed?

The terrain flight mode defined by being as close to the surface as possible while varying airspeed is Nap Of the Earth (NOE) flight. This flight technique is utilized primarily to enhance flight safety by avoiding detection and reducing exposure to threats. NOE flight allows pilots to navigate close to the terrain, using the natural landscape for cover and concealment. By flying at low altitudes with varying airspeeds, the aircraft can adapt to the terrain's changes, making it more difficult for adversaries to predict its trajectory.

In this context, the other flight modes do not possess the same characteristics. Cruise flight typically occurs at a steady altitude and speed, which does not match the dynamic nature of NOE. Hovering flight implies a stationary position in the air, lacking the movement across variable terrain, and high altitude flight operates well above ground level, eliminating the close proximity that NOE embraces. Thus, Nap Of the Earth flight is specifically defined by its low altitude and adaptability in airspeed, making it distinct amongst the options given.