Linkage isomerism is shown only by ambidentate ligands, which have two different donor atoms but coordinate through just one of them at a time. The nitrite ion is a classic ambidentate ligand: when it bonds through nitrogen it is called nitro and gives a yellow complex, and when it bonds through oxygen it is called nitrito and gives a red complex. Thus [Co(NH3)5(NO2)]^{2+} exists as nitro and nitrito linkage isomers with different colours and stabilities. Ionisation isomerism is wrong because no anion is being exchanged with the outside of the sphere. Optical isomerism is irrelevant since the species need not be chiral and the difference here is the donor atom, not handedness. Coordination isomerism requires both ions to be complexes, which is not the situation. This nitro-nitrito pair is the standard NCERT illustration of linkage isomerism. Plausibility check: the observable colour change from yellow to red on switching the donor atom directly confirms that two genuinely different bonding modes exist, validating linkage isomerism.