$Na, Mg, Al, Si, P, S, Cl$ and $Ar$ are the elements of the third period.
$(a)$ Which one of them is an inert gas? Why?
$(b)$ Which one of them has a valency of $4$? What kind of bond does it form: ionic or covalent?
$(c)$ Which of them are electropositive? Pick any two.

(N/A) Argon $(Ar)$ is an inert gas because its outermost shell is completely filled with $8$ electrons,making it stable.
$(b)$ Silicon $(Si)$ has a valency of $4$. It forms covalent bonds because it shares its $4$ valence electrons to complete its octet.
$(c)$ Sodium $(Na)$ and Magnesium $(Mg)$ are electropositive elements because they have a tendency to lose electrons to attain a stable noble gas configuration.