What are the optimal conditions for carburetor icing?

The optimal conditions for carburetor icing occur when temperatures drop below 70°F and relative humidity exceeds 80%. This phenomenon is primarily due to the decrease in temperature that occurs when fuel vaporizes in the carburetor. As fuel expands and vaporizes, it absorbs heat from the surrounding air, leading to a drop in temperature that can precipitate icing when humidity is high.

High humidity levels provide an ample supply of moisture in the air, which can condense and freeze on the cold surfaces within the carburetor, obstructing fuel flow and affecting engine performance. Thus, when combined, these conditions create a perfect environment for carburetor icing to occur, making option A the correct choice.

In contrast, the other conditions described in the other options do not facilitate carburetor icing effectively. For instance, higher temperatures or lower humidity diminish the likelihood of vapor condensation and freezing, preventing the accumulation of ice in the carburetor system.