The correct answer is C) Natural supply and mechanical exhaust. The reason this is the correct answer is that the regulations for small passenger vessels require enclosed spaces containing gasoline engines or gasoline fuel tanks to have natural supply ventilation and mechanical exhaust ventilation. This ensures adequate airflow to prevent the buildup of flammable vapors. The natural supply ventilation provides fresh air intake, while the mechanical exhaust actively removes the potentially hazardous vapors to the exterior of the vessel. The other options are incorrect because A) only requires one exterior opening, which may not provide sufficient airflow, and B) and D) do not accurately describe the required ventilation system for these types of enclosed spaces on small passenger vessels.