Irregular motion of an aircraft in flight caused by variations in wind velocities is called ____.

The correct answer is identified as "Air turbulence." This term specifically refers to the irregular motion of an aircraft in flight caused by variations in wind velocities. Turbulence can occur as a result of factors such as atmospheric pressure changes, jet streams, and natural obstacles like mountains. Understanding turbulence is crucial for pilots and air traffic controllers, as it can significantly affect flight safety and comfort.

The other terms may suggest similar concepts but may not accurately capture the widely accepted terminology in aviation. For example, while "flight turbulence" conveys a sense of movement during the flight, it is not the standard phrase used in the context of aviation. "Aircraft turbulence" emphasizes the aircraft itself but does not account for the environmental factors contributing to the phenomenon. "Structural turbulence" implies a focus on the physical structure of the aircraft, which is not relevant to the description of wind variations affecting flight. Therefore, "air turbulence" is the most accurate term to define the irregular motion caused by fluctuating wind conditions.