Explanation

(c)
Since the complexes $\left\lbrack {PtCl}_{2}\left( {NH}_{3} \right)_{4} \right\rbrack{Br}_{2}\ and\ \lbrack{PtBr}_{2}\left( {NH}_{3} \right)4{Cl}_{2}$have the same molecular formula but on ionisation they give different ions, they exhibit ionisation isomerism.
$\ \left\lbrack {PtCl}_{2}\left( {NH}_{3} \right)_{4} \right\rbrack{Br}_{2} \rightleftharpoons$ $\left\lbrack {PtCl}_{2}\left( {NH}_{3} \right)_{4} \right\rbrack^{2 +}\ \ + 2{Br}^{-}$
$\left\lbrack {PtBr}_{2}\left( {NH}_{3} \right)_{4} \right\rbrack{Cl}_{2}$ $\rightleftharpoons \left\lbrack {PtBr}_{2}\left( {NH}_{3} \right)_{4} \right\rbrack^{2 +} + 2{Cl}^{-}$