Combustion is best defined as:

The definition of combustion is fundamentally linked to the chemical reaction that occurs when a fuel reacts with oxygen, producing heat and light. Hence, the most accurate definition of combustion is the chemical reaction between oxygen and flammable material.

The first option describes this interaction effectively by emphasizing the necessity of both oxygen and a fuel source to initiate the chemical reaction characteristic of combustion. This definition highlights the requirement for a flammable material, which is essential in understanding the combustion process, and why certain materials are considered combustible.

While the other options touch on important aspects of fire behavior, they don't capture the essence of what combustion specifically entails. The second option, referring to self-sustaining combustion, describes a particular condition (like ignition or the sustaining of a fire) rather than defining combustion itself. The cooling process mentioned in the third option is not associated with combustion but rather with fire suppression. Lastly, the ignition point of solid fuels pertains to the temperature at which a solid fuel will ignite, which is important for understanding fire dynamics but does not define combustion itself.

Thus, the defining characteristic of combustion remains the chemical reaction involving oxygen and fuel, providing the basis for understanding fire and its behavior.