Answer;
Ar (Argon)
Explanation;
-S (Sulfur) atom has 16 electrons, however it forms an anion (negatively charged ion) by gaining two electrons. Therefore; S^2- has 18 electrons. In other words it requires two electrons to form a stable configuration or an octet configuration.
-Argon (Ar) is a noble gas (group 8 element) which means it has an octet configuration. It has 18 electrons similar to an ion of sulfur (S^2-).