Combustion can be defined as:

Combustion is fundamentally a chemical reaction that occurs when oxygen reacts with combustible materials, such as fuels. This process typically produces heat and often light, resulting in the release of energy. The correct definition reflects the essence of combustion as a chemical reaction, where the oxygen combines with the elements of the fuel, leading to the formation of new substances, primarily carbon dioxide and water vapor, along with the release of energy in the form of heat.

The other definitions do not accurately capture the holistic nature of combustion. For instance, mixing chemicals with air does not necessarily mean combustion is occurring; the presence of oxygen is key, but without the necessary conditions of heat and fuel, mixing alone does not lead to combustion. Transforming matter from one state to another is a broader concept applicable to many physical and chemical processes but does not specify the reactive nature of combustion. Lastly, heating fuel without any chemical change describes a physical process rather than a chemical reaction, which is precisely what combustion is defined as—an active chemical transformation driven by the interaction of fuel and oxygen.