Each complex is examined for the kind of isomerism its formula allows.
Step 1: Complex (i). In \([\text{Co(NH}_3)_5(\text{NO}_2)](\text{NO}_3)_2\) the nitrite ion is ambidentate. When it donates through N the ligand is called nitro (\(-\text{NO}_2\)) and the isomer is \([\text{Co(NH}_3)_5(\text{NO}_2)]^{2+}\); when it donates through O it is called nitrito (\(-\text{ONO}\)) giving \([\text{Co(NH}_3)_5(\text{ONO})]^{2+}\). Because the same ligand attaches through different donor atoms, this is linkage isomerism.
Step 2: Complex (ii). \([\text{Cr(NH}_3)_6][\text{Co(CN)}_6]\) has a complex cation and a complex anion. Swapping the sets of ligands between the two metals produces \([\text{Co(NH}_3)_6][\text{Cr(CN)}_6]\). Since both partners are complexes and their ligands are exchanged, this is coordination isomerism.
Step 3: Complex (iii). \([\text{Fe(NH}_3)_2(\text{CN})_4]^-\) is octahedral of type \(\text{MA}_2\text{B}_4\). The two ammonia groups can sit next to each other (cis) or across from each other (trans), so it shows geometrical isomerism. The cis form has the two \(\text{NH}_3\) at \(90^\circ\) and the trans form has them at \(180^\circ\).
\[\boxed{\text{(i) linkage; (ii) coordination; (iii) cis-trans geometrical}}\]