The correct option is B [Cr(H2O)4Cl2](NO2)
Ionisation isomers are complexes that produces different ions in solution, i.e., they have ions interchanged inside and outside the coordination sphere.
[Cr(H2O)4Cl(NO2)]Cl and [Cr(H2O)4Cl2]NO2 have different ions inside and outside the coordination sphere and hence are ionisation isomers.