Here's how to determine the noble gas isoelectronic with a sulfur ion:

* Sulfur's Atomic Structure: Sulfur (S) has 16 electrons.

* Sulfur Ion (S²⁻): When sulfur gains two electrons to form the sulfide ion (S²⁻), it now has 18 electrons.

* Isoelectronic: Isoelectronic species have the same number of electrons.

* Noble Gas with 18 Electrons: Argon (Ar) has 18 electrons.

Therefore, the noble gas isoelectronic with a sulfur ion is argon (Ar).