Which of the elements from above (S, Cu, Ar) are considered unreactive? (Explain why using the concept of valence electrons)

Ar

Explanation:

Valence electrons in an atom or element play important role in making an element reactive or unreactive. Valence electrons are the electrons present in the outermost shell of the atom that participates in a chemical reaction and forms a chemical bond with other atoms. Elements having no valence electrons are stable and acts unreactive.

Argon or Ar is the unreactive element from the given elements because there are no valence in argon and all the three electrons shells are full. Electronic configuration of Ar is [Ne]3s23p6 which shows the stability of argon.

Hence, the correct answer is "Ar".

it's all four options.