The COORDINATION NUMBER of the central metal in a complex is

1. NCERT §5.2 defines coordination number (CN) explicitly. 2. CN is the NUMBER OF LIGAND DONOR ATOMS that are directly bonded to the central metal — not the number of ligand molecules, in case of polydentate ligands. 3. Example: $[\mathrm{Co(NH_3)_6}]^{3+}$ has six ammonia ligands, each contributing one nitrogen — CN = 6. 4. Example: $[\mathrm{Co(en)_3}]^{3+}$ has three ethylenediamine ligands (bidentate, each contributing two nitrogens) — CN = 6 as well. 5. Common CN values: 2, 4, 6 (most common for d-block), with some examples up to 8. 6. Option A is the complex charge, not the CN. Option C is the oxidation state (a separate property). Option D is the atomic number (irrelevant to coordination). _Source: NCERT Class 12 Chemistry Part 1, Ch 5, §5.2 (Definitions — coordination number), p. 2–3._