Final answer:
SF₆ is the molecule with an expanded octet, as the sulfur atom forms six covalent bonds, exceeding the typical octet rule.
Step-by-step explanation:
The molecule that contains an atom with an expanded octet is SF₆. In SF₆, the central sulfur atom (S) forms six covalent bonds with six fluorine (F) atoms, thus accommodating more than eight electrons in its valence shell. This is possible for sulfur because it resides in the third period of the periodic table, where elements have d orbitals that can hold extra electrons beyond the octet. In contrast, the other molecules listed - H₂O, CH₄, and CO₂ - have central atoms that typically follow the octet rule, meaning they do not have expanded octets.