Explanation

Fuel Cell Electric Vehicles (FCEVs) utilize fuel cells where onboard hydrogen gas combines electrochemically with oxygen from the ambient air to generate electricity:

• (a) Hydrogen peroxide (Incorrect): A reactive compound not formed as the functional end-product of automotive fuel cell operations.

• (b) Hydronium (Incorrect): A hydrated proton species present in acidic solutions, not an exhaust gas emitted from tailpipes.

• (c) Oxygen (Incorrect): Consumed as an intake reactant from ambient air at the cathode, not an emitted byproduct.

• (d) Water vapour (Correct): The electrochemical combination of hydrogen and oxygen produces only electricity, heat, and pure water, resulting in water vapour being the sole tailpipe emission.

Hence, Option (d) is the correct answer (Water vapour).