When is prevailing visibility reported as variable?

Prevailing visibility is reported as variable when there are rapid fluctuations in visibility that meet specific criteria. The correct choice indicates that this variability occurs when visibility rapidly increases and decreases by 1/2 mile or more while being less than 3 miles. This definition captures the essence of what it means for visibility to be described as variable, as it highlights the significance of both the amount of fluctuation and the visibility distance itself.

In this context, if the visibility were consistently at a single value—whether it's 3 miles or any other stable measurement—there would be no variability to report. Similarly, simply stating that observed objects are less than 2 miles away does not provide adequate context for fluctuating visibility, as it doesn't reflect the criteria related to rapid changes in distance that define "variable" visibility. Thus, the conditions outlined in the correct answer encapsulate the necessary elements that would lead to a report of variable prevailing visibility.