Which reported changes in prevailing visibility require a SPECI to be taken?

The correct answer focuses on the significant reductions in prevailing visibility that warrant a Special Weather Report (SPECI). A change from 2 1/2 miles to 2 miles, or from 2 miles to 1 mile, indicates diminishing visibility under the critical 3-mile threshold. These reductions are particularly significant because they can substantially impact pilot operations, flight safety, and navigation. Situations with lower visibility are always important for air traffic control and flight management, making timely reporting crucial to ensure that all relevant parties are aware of the changing conditions.

In contrast, the other options either represent increases in visibility or do not reflect reductions that reach below the 3-mile threshold, which is a critical point for aviation operations. Changes that go from 3 miles to 5 miles or from 1 mile to 2 miles, for example, are improvements in visibility and typically do not trigger a SPECI report. The importance here lies in understanding that SPECI reports are required for certain thresholds of reduced visibility, particularly for those conditions that might affect the safety and efficiency of flight operations.