Explanation

(b)
As cobalt is present as ${CO}^{3 +}$ and coordination number of cobalt is 6, the molecular formula of compound should be ${CoCl}_{3}$. ${yNH}_{3}$ . Now, as it gives a total of three ions when dissolved in water, its structural formula must be
$\left\lbrack CoCl\left( {NH}_{3} \right)_{5} \right\rbrack{Cl}_{2}$
$\left\lbrack CoCl\left( {NH}_{3} \right)_{5} \right\rbrack{Cl}_{2} \rightleftharpoons \left\lbrack CoCl\left( {NH}_{3} \right)_{5} \right\rbrack^{2 +} + 2{Cl}^{-}$
Thus, only one $Cl^{-}$ ion is satisfying both primary and secondary valency of ${Co}^{3 +}$ in this compound.