The correct answer is D) They are heavier than air and will settle in the lowest part of the vessel. Gasoline vapors are heavier than air, so they will settle in the lowest part of the vessel rather than rising to the highest part. This is an important safety consideration, as gasoline vapors are highly flammable and can accumulate in low areas, creating a serious fire hazard. The regulations require that gasoline vapors be properly vented to prevent this buildup and potential ignition. The other options are incorrect because: A) Gasoline vapors are heavier than air, not lighter. B) Venting gasoline vapors into the engine is unsafe and improper. C) Venting into the wheelhouse could expose crew to the hazardous vapors.