The extra stability of complexes formed by polydentate ligands compared with similar monodentate ligands is called the chelate effect. When one bidentate ethylenediamine replaces two separate ammonia molecules, the number of free particles in solution increases because fewer ligand molecules are needed to occupy the same sites, releasing previously bound species. This rise in the number of free molecules raises the entropy of the system, making the free-energy change more negative and the chelate complex more stable. The claim that ammonia is a stronger field ligand is false; ethylenediamine actually lies higher in the spectrochemical series and forms stable five-membered rings. Ethylenediamine is a neutral ligand, so the charge argument is wrong. The oxidation state of the metal is unchanged when ligands are swapped, so that option fails. The chelate effect is exactly as discussed in NCERT stability sections. Plausibility check: a more positive entropy term lowering free energy is fully consistent with the experimentally larger stability constants measured for ethylenediamine complexes.