Answer:
Ne.
Step-by-step explanation:
Neutral argon, atomic number 18, has 18 electrons.
Cl⁻:
Neutral atom of Cl, atomic number 17, has 17 electrons.
When it gains electron and be Cl⁻, then it has 18 electrons a neutral Ar.
S²⁻:
Neutral atom of S, atomic number 16, has 16 electrons.
When it gains 2 electrons and be S²⁻, then it has 18 electrons a neutral Ar.
K⁺:
Neutral atom of K, atomic number 19, has 19 electrons.
When it losses electron and be K⁺, then it has 18 electrons a neutral Ar.
Ca²⁺:
Neutral atom of Ca, atomic number 20, has 20 electrons.
When it losses 2 electrons and be Ca²⁺, then it has 18 electrons a neutral Ar.
Ne:
It is a noble gas that has 10 electrons.
So, the right choice is: Ne.