Correct Answer: Industrial purposes

Solution : The correct answer is (D) industrial purposes.

Commercial energy sources, which include conventional energy sources like fossil fuels (coal, oil, natural gas), are primarily utilized in industries for various purposes such as manufacturing, production, and operation of machinery and equipment. These energy sources provide the necessary power and fuel for industrial processes, including heating, cooling, transportation of goods, and running industrial machinery. While commercial energy sources may also be used in other sectors like residential heating, agricultural irrigation, and public transportation, their primary focus and usage are in industrial applications.