Ionisation isomerism appears when two complexes have the same overall formula but exchange an anion between the coordination sphere and the counter-ion position, so they release different ions in solution. The pair here is [Co(NH3)5Br]SO4 and [Co(NH3)5SO4]Br. The first has free sulphate outside the sphere, so it gives a white precipitate of barium sulphate with barium chloride. The second has free bromide outside the sphere, so it gives a pale yellow precipitate of silver bromide with silver nitrate. Because they ionise to give different anions, they are ionisation isomers. Linkage isomerism would require an ambidentate ligand binding through different atoms, which is not the case. Coordination isomerism needs both cation and anion to be complex ions. Hydrate isomerism involves water shifting in and out of the sphere, but no water exchange is described here. This matches the NCERT definition of ionisation isomerism. Plausibility check: each isomer yields a distinct, predictable precipitate corresponding to the anion it frees, confirming the assignment.