The correct option is B 10
The complex [Co(NH3)4(NO2)2](NO3)] has ten isomers. They include geometrical, ionization and linkage isomers.
NO3 and NO2 groups can be either inside or outside the coordination sphere. This gives ionization isomers. NO2 group can coordinate through N or O atom. This gives linkage isomerism. cis and trans isomers give geometrical isomers.