The correct choice that only includes terms that describe oxygen gas (O2) is: atom, compound, molecule.
However, please note that the best answer from a conceptual standpoint would actually be molecule, pure substance, but that is not one of the options provided. The term "element" generally applies to the individual types of atoms, not the gas itself, which is molecular (O2 is a diatomic molecule). Therefore, "compound" would not be appropriate either since O2 is not a compound but an elemental gas.
Given your options, the closest terms describing oxygen gas specifically would still fit in the category of being a molecule and a pure substance.
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