The correct answer is B) heavier than air. The vapors given off by heated fuel oil are flammable and explosive, and they are also heavier than air. This means that the vapors will sink and accumulate in low-lying areas, creating a potentially dangerous situation. The other options are incorrect - the vapors are not odorless (they have a distinct fuel-like smell), they are not lighter than air (which would cause them to rise), and they are not non-toxic (the vapors can be harmful if inhaled).