Which of the following is a product of complete combustion of hydrocarbons?

• A. Carbon monoxide and water
• B. Water vapor
• C. Carbon dioxide and water
• D. Soot

Answer: (C)

When a hydrocarbon burns with plenty of oxygen, all the carbon ends up as carbon dioxide and all the hydrogen becomes water. This is complete combustion: products are CO2 and H2O (water can be liquid or vapor depending on temperature). Among the options, carbon dioxide and water is the only pairing that matches this outcome. The others show products that occur when combustion is incomplete (carbon monoxide and water, or soot) or would not reflect the carbon from the fuel.