1. Identify the metal ion and ligands

All the complexes are of the type $[\text{Co}(\ldots)]Cl_2$, so the complex cation is $[\text{Co}(\ldots)]^{2+}$. Since $Cl^-$ is outside the coordination sphere, cobalt is in the $+2$ oxidation state in each case.

The ligands are:

• $NH_3$ = monodentate ligand
• en $=$ ethylenediamine $= H_2NCH_2CH_2NH_2$ = bidentate ligand

2. Use the chelate effect

Complex stability increases strongly when monodentate ligands are replaced by polydentate ligands like en. This is called the chelate effect.

Reason:

• Bidentate ligands form rings with the metal.
• More chelate rings generally give greater thermodynamic stability.
• Entropy also favors chelation.

3. Count the number of en ligands in each complex

• A: $[Co(en)(NH_3)_4]Cl_2$
  Contains 1 en ligand
• B: $[Co(en)_3]Cl_2$
  Contains 3 en ligands
• C: $[Co(en)_2(NH_3)_2]Cl_2$
  Contains 2 en ligands
• D: $[Co(NH_3)_6]Cl_2$
  Contains 0 en ligands

Thus, stability due to chelation should increase in the order: $[Co(NH_3)_6]^{2+} < [Co(en)(NH_3)_4]^{2+} < [Co(en)_2(NH_3)_2]^{2+} < [Co(en)_3]^{2+}$

4. Most stable complex

The complex with the maximum number of chelate rings is: $[Co(en)_3]Cl_2$

So the most stable complex is Option B.

5. Compare with stored correct answer

Stored correct answer = B

This matches the derived answer.