Explanation

(a)
In $\left\lbrack CoCl_{4} \right\rbrack^{2 -}$ ion, central metal atom i.e., cobalt is in +2 oxidation state. Hence,
_{27}Co=[Ar]$3d^{7\ }4s^{2}$
$\therefore$ _{27}Co^{2+}=[Ar]$3d^{7}$
[table] ⥮ | ⥮ | $\upharpoonleft$ | $\upharpoonleft$ | $\upharpoonleft$ [/table]
Hence, number of unpaired electrons is three as Cl is weak ligand.