* Molecules are formed when two or more atoms bond together.

Let's look at the options:

* A. NaCl (Sodium Chloride): This is an ionic compound, not a molecule. Ionic compounds are formed by the electrostatic attraction between positively and negatively charged ions.

* B. He (Helium): This is an atom, not a molecule. Atoms are the basic building blocks of matter.

* C. H2O (Water): This is a molecule. It is formed by two hydrogen atoms bonded to one oxygen atom.

* D. Li (Lithium): This is an atom, not a molecule.

Therefore, only H2O represents a molecule.