Many biological molecules function as coordination complexes in which a metal ion is held by a polydentate organic ligand. In haemoglobin the active haem group is a porphyrin ring that coordinates a central iron(II) ion through four nitrogen donor atoms in a roughly planar arrangement. A fifth position binds a histidine residue of the protein, and the sixth coordination site is left free to bind and release molecular oxygen reversibly, enabling oxygen transport in blood. The iron must remain in the +2 state for reversible oxygen binding; oxidation to iron(III) gives methaemoglobin, which cannot carry oxygen. Magnesium(II) is the central ion of chlorophyll, not haemoglobin, so that option is wrong. Cobalt(III) is found in vitamin B12, a different biological complex. Zinc(II) occurs in enzymes such as carbonic anhydrase but not in haem. This example is highlighted in NCERT as an important application of coordination chemistry. Plausibility check: the requirement for reversible, non-destructive oxygen binding is consistent only with labile iron(II), confirming the assignment.