Correct Answer: Hydrogen

Solution : The correct answer is Hydrogen.

Hydrogen is the most eco-friendly of all the fuels. When hydrogen is used as fuel, it produces water vapour as the primary by-product, which is environment-friendly. Unlike coal, kerosene and diesel, Hydrogen combustion doesn't release hazardous pollutants, such as particulate matter, sulphur dioxide, nitrogen oxides or greenhouse gases. Therefore, Hydrogen can significantly reduce pollution and contribute to more sustainable energy systems.