Observe the different pairs of isomeric coordination compounds...

In pair A, the ligand $\mathrm{NO_2^-}$ is an ambidentate ligand which can bind through either nitrogen or oxygen. This represents linkage isomerism. (A $\rightarrow$ 3)

In pair B, the complexes differ in the number of water molecules present as ligands inside the coordination sphere and as water of crystallization outside. This represents solvate (or hydrate) isomerism. (B $\rightarrow$ 4)

In pair C, the counter ion and a ligand exchange places, producing different ions in solution ($\mathrm{Br^-}$ vs $\mathrm{SO_4^{2-}}$). This represents ionization isomerism. (C $\rightarrow$ 1)

In pair D, there is an interchange of ligands between the cationic and anionic coordination entities. This represents coordination isomerism. (D $\rightarrow$ 2)

The correct matching is A-3, B-4, C-1, D-2.

Answer: A-3, B-4, C-1, D-2