What is a common contaminant that can affect fuel quality?

Water is a common contaminant that can significantly affect fuel quality. When water contaminates fuel, it can lead to a variety of issues such as microbial growth, which can create sludge and cause corrosion in storage tanks and engines. This contamination can also lead to poor engine performance, fuel instability, and even damage to fuel delivery systems.

Additionally, the presence of water can result in the separation of fuel components, which compromises the integrity and effectiveness of the fuel. Water is particularly concerning in hydrocarbon-based fuels because it can change the physical and chemical properties of the fuel, ultimately affecting its combustion efficiency.

In contrast, the other options—oxygen, carbon dioxide, and helium—do not typically contaminate fuel to the same detrimental extent. While oxygen can contribute to oxidation and degradation of fuel over time, it does not act in the same immediate way as water does in terms of direct contamination. Carbon dioxide is generally a product of combustion rather than a contaminant, and helium is an inert gas that does not impact fuel quality at all.