Denticity counts the number of donor atoms of a single ligand that simultaneously bind to the central metal. The ethylenediaminetetraacetate ion (EDTA^{4-}) possesses two nitrogen atoms from the ethylenediamine backbone and four oxygen atoms from the four deprotonated carboxylate groups, giving a total of six donor atoms. All six can coordinate to one metal centre, wrapping around it and forming an exceptionally stable octahedral chelate, which is why EDTA is hexadentate. This multiple-ring chelation is responsible for the strong sequestering action that removes Ca^{2+} and Mg^{2+} from hard water. The bidentate option fits only ethylenediamine alone, not the full EDTA. Tetradentate would ignore the two amine nitrogens. Monodentate is far too low and applies to ligands like chloride or ammonia. The chelate effect, central to NCERT's discussion of stability, explains EDTA's clinical and analytical use. Plausibility check: a hexadentate ligand neatly fills all six positions of an octahedral metal, consistent with EDTA forming 1:1 complexes with most divalent and trivalent cations.