The correct answer is C. element, molecule, pure substance.
Explanation:
- Oxygen gas (O2) consists of two oxygen atoms bonded together, making it a molecule.
- It is an elemental form of oxygen, as it consists only of oxygen atoms and is one of the basic elements on the periodic table.
- As a gas that has a consistent composition and does not vary, it is also considered a pure substance.
The other options contain terms that do not accurately describe oxygen gas. For example, option B includes "compound," which is incorrect as O2 is not a compound but an element.