Final answer:
O2 is a molecular element because it consists of only one type of atom - oxygen. It is called a diatomic molecule, as opposed to a compound, which would contain two or more different elements. Therefore, the correct option is B.
Step-by-step explanation:
Oxygen gas, represented by the molecular formula O2, is a diatomic molecule as it consists of two oxygen atoms bonded together. It is classified as a molecular element, not a compound, because it is composed of only one type of element; all atoms in the molecule are oxygen atoms. This differs from compounds, which are composed of two or more different elements chemically combined in a fixed ratio.
Iron (Fe) is an element that is represented without any subscript, indicating an atomic element. Iron oxide (Fe2O3), on the other hand, is a compound because it contains two different elements, iron and oxygen, in a definite proportion. One particle of iron oxide would be considered a molecule since it consists of atoms of different elements bonded together.