Final answer:
The correct answer is Na or Sodium. It is not a molecule but an atom, unlike the other options O₂, O₃, and H₂O which are molecules comprising of multiple atoms bonded together. Therefore the correct option is C).
Step-by-step explanation:
The statement, 'which of the following is not a molecule?' concerns molecules and the fundamental nature of chemistry. A molecule is formed when two or more atoms join together chemically. For the given question, options a (O₂), b (O₃), and d (H₂O) actually are examples of molecules because they consist of multiple atoms bonded together.
However, option c (NA), presumably should be 'Na', which stands for Sodium, is not a molecule but an atom. Sodium belongs to the category of chemical elements and is represented by a single atom. Thus, Na or Sodium being an atom, not a molecule, is the correct answer to the student's question.
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