Which of the following is one of the three terrain flight modes?

The terrain flight mode known as "Nap of the Earth" involves flying at very low altitudes, typically just above the natural terrain features, in order to minimize visibility to enemy forces and reduce the aircraft's exposure to threats. This mode leverages the natural contours of the landscape—such as hills, trees, and buildings—to provide cover and concealment while navigating. This tactic is crucial for flight safety and operational effectiveness, especially in combat situations.

Other flight modes, such as high altitude, emphasize flying at greater heights, often for different tactical advantages, and do not involve the same level of concealment offered by low-level flying. Therefore, "Nap of the Earth" stands out as a specific and defined mode of terrain flight that enhances stealth and operational success.