Oxidation is best described as:

Oxidation is best described as a chemical change that occurs when a substance combines with oxygen. This process is fundamental in various chemical reactions, particularly in combustion, where fuel reacts with oxygen to produce heat and light. The combination of oxygen with other elements or compounds typically results in the formation of oxides, which are new substances with different properties from the reactants. Understanding oxidation is crucial in firefighting as it directly relates to how fires start and propagate, fundamentally involving the reaction of fuel with oxygen in the environment.

The other choices, while related to reactions and transformations, do not accurately capture the essence of oxidation. A rapid reaction between fuel and air describes combustion, which encompasses oxidation but is not the full definition. Slow decomposition would refer more to processes like rusting or other forms of corrosion, which can be classified under oxidation but do not represent the complete concept. Lastly, a physical transformation pertains to changes in state (like solid to liquid) without altering the chemical structure of a substance, which is not related to the definition of oxidation.