IIT-JEE 2010 · Paper 1, Chemistry question 2

Ionisation isomers swap a ligand inside the coordination sphere with the counter-ion outside, so they give different ions in solution.

In $\ce{[Cr(H2O)4Cl(NO2)]Cl}$ the counter-ion is $\ce{Cl-}$ and $\ce{NO2-}$ is coordinated. Exchanging them gives $\ce{[Cr(H2O)4Cl2]NO2}$.

(C) is a linkage isomer (ONO vs NO₂); (A) and (D) do not have the same formula.

(B)