The correct answer is B) heavier than air. The vapors given off by heated fuel oil are flammable and explosive because they are hydrocarbon-based. These vapors are also heavier than air, meaning they will sink and accumulate in lower areas rather than dissipating. This is an important safety consideration, as heavier-than-air vapors can create a fire or explosion hazard in enclosed or low-lying spaces. The other answer choices are incorrect - the vapors are not lighter than air, non-toxic, or odorless. Understanding the physical properties of flammable fuel oil vapors is crucial for proper handling and safety procedures on vessels, as required by the US Coast Guard for captain's license examinations.