Final answer:
SF6 is the compound where the octet is expanded to include 12 electrons due to the sulfur atom forming six covalent bonds with fluorine atoms.
Step-by-step explanation:
In the compounds listed, SF6 is the one where the octet is expanded to include 12 electrons. In SF6, the central sulfur atom forms six covalent bonds with six fluorine atoms. Sulfur is in the third period of the periodic table and has access to d orbitals, which allows it to accommodate more than eight electrons in its valence shell, hence allowing for an expanded octet. The other compounds listed do not have this expanded octet: H2S and PCl3 follow the normal octet rule, and while PCl5 also has an expanded octet, it has 10 electrons in its valence shell and not 12 like SF6.