Which one of the following is the exhaust pipe emission from Fuel Cell Electric Vehicles, powered by hydrogen?

Water vapour

In a hydrogen fuel-cell electric vehicle, hydrogen reacts electrochemically with oxygen from the air to generate electricity. The principal reaction product is water, which is released as water vapour or water through the vehicle's exhaust. Hydrogen peroxide is not the normal exhaust product. Hydronium is an ion involved in certain chemical processes, not the vehicle's exhaust emission. Oxygen is a reactant consumed in the fuel cell rather than the principal emission. Hence, option D is correct.