The complexes [PtCl2(NH3)4]Br2 and [PtBr2(NH3)4]Cl2 are examples for isomerism:
Ionisation Isomers are those isomers where the isomers can be thought of as occurring because of the formation of different ions in solution.
In [PtCl2(NH3)4]Br2,Br− in solution and in another isomer [PtBr2(NH3)4]Cl2,Br− anion in solution are examples of ionisation isomers.
Hence, option C is correct.