Which of the following gases is produced when hydrogen is used as a fuel?

When hydrogen is used as a fuel, the primary product of its combustion is water. This reaction occurs when hydrogen gas reacts with oxygen gas, resulting in the formation of water vapor. The chemical equation for this reaction can be represented as:

2 H₂(g) + O₂(g) → 2 H₂O(g)

In this process, hydrogen combines with oxygen, and because hydrogen is a clean fuel, it does not produce harmful emissions like carbon dioxide or nitrogen oxides that are typical when fossil fuels are burned. The efficiency and environmentally friendly aspect of hydrogen combustion make water the main byproduct, aligning with the choice provided.

While other gases listed can be produced in different contexts (such as nitrogen oxides from high-temperature combustion processes), they do not form during the direct combustion of hydrogen with oxygen. Thus, water is recognized as the significant and expected product when hydrogen is utilized as a fuel source.