Which of the following is a byproduct of burning a log?

When a log is burned, it undergoes a chemical reaction known as combustion. During this process, the organic materials in the wood react with oxygen from the air, producing heat, light, carbon dioxide, and water vapor. Water vapor is a natural byproduct of this reaction, as the hydrogen and oxygen components of the wood molecules combine to form H2O in the gaseous state.

The other choices do not result from the combustion of wood. While burning wood produces energy, it does not directly generate electricity unless it is part of a larger system designed for that purpose. Plastic is a synthetic material that cannot be derived from burning wood, and metal is a solid substance that is not produced in the combustion of a log. Therefore, water vapor is the correct answer, as it is a direct result of burning wood in a fire.