Hydrogen fuel cell vehicles produce one of the following as 'exhaust'

Hydrogen fuel cell vehicles generate electrical power by an electrochemical reaction in which hydrogen reacts with oxygen; the primary measurable tail‑pipe/byproduct is water (usually emitted as water vapour) and heat. Text sources explicitly state that fuel‑cell vehicles produce water vapour as their only exhaust emission and have near‑zero pollution compared with internal combustion engines [1]. Official technical and government resources likewise note that FCEVs emit no harmful tailpipe pollutants and only release water vapour and warm air as byproducts of the fuel cell reaction [2]. Therefore NH3, CH4 and H2O2 are not typical fuel‑cell exhausts; H2O is the correct option.

Sources

1. [1] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 22: Renewable Energy > Fuel cells for automobile transport > p. 296
2. [2] https://afdc.energy.gov/vehicles/fuel-cell