Combustion of fossil fuels is defined as:

• A. Evaporation of water to form steam
• B. Absorption of heat without chemical change
• C. Chemical reaction between a fuel and oxygen that yields carbon dioxide and water and releases energy
• D. Nuclear fission of heavy elements

Answer: (C)

Combustion is an oxidation reaction where a fuel reacts with oxygen to release energy. When fossil fuels, which are mainly hydrocarbons, burn in air with sufficient oxygen, they convert to carbon dioxide and water and release energy as heat (and often light). This description precisely matches what happens during combustion: a chemical reaction with oxygen producing CO2 and H2O and releasing energy. The other statements describe processes that aren’t chemical combustion—evaporation is simply a phase change, absorbing heat without changing chemical composition is heating, and nuclear fission is a nuclear, not a chemical, process.