$(a)$ Amongst the following elements,identify the ones that would form anions:
$K, O, Na, F, Ca, Cl, Mg$
$(b)$ Write the electronic configuration of the anions identified above.

(A) Elements that form anions are those that gain electrons to complete their valence shell. Among the given elements,$O$ (Oxygen),$F$ (Fluorine),and $Cl$ (Chlorine) are non-metals that gain electrons to form anions.
$(b)$ The electronic configurations of the identified anions are:
$O^{2-}$ (Oxide ion): Atomic number $8$,configuration $2, 6$. By gaining $2$ electrons,it becomes $2, 8$.
$F^{-}$ (Fluoride ion): Atomic number $9$,configuration $2, 7$. By gaining $1$ electron,it becomes $2, 8$.
$Cl^{-}$ (Chloride ion): Atomic number $17$,configuration $2, 8, 7$. By gaining $1$ electron,it becomes $2, 8, 8$.