What characteristic of fuel systems helps prevent contamination during refueling?

A closed system design in fuel systems is essential for preventing contamination during refueling. This design minimizes the exposure of the fuel to the outside environment, which can introduce dirt, water, or other contaminants. In a closed system, fuel is transferred from storage to the aircraft without being exposed to atmospheric conditions, thus reducing the chances of particles or moisture entering the fuel system.

Other aspects like color-coded fuel hoses and high-visibility refueling points aid in the management and identification of fuel types, which is important for safety and operational efficiency, but they do not directly prevent contamination. Additionally, personnel training programs are crucial for ensuring that staff understands proper refueling procedures, yet they alone cannot prevent contaminants from entering the system if the physical design of the fuel system does not support this goal.