The IUPAC name of $[\mathrm{Co(NH_3)_6}]^{3+}$ is

1. Naming a coordination complex: ligands first (in alphabetical order), then central metal, oxidation state in Roman numerals. 2. Number of ligands: prefix is 'hexa' for six ammonia. 3. Ligand name: NH$_3$ as a ligand is called 'AMMINE' (note the double-m). 4. Metal name: since the complex is CATIONIC, the metal keeps its English name 'cobalt' (no '-ate' suffix). For anionic complexes, '-ate' would apply (e.g. cobaltate). 5. Oxidation state: as computed in earlier MCQs, Co is +3, written as (III). 6. Assemble: 'hexaamminecobalt(III) ion' — option B. 7. Option A uses '-ate' (only for anionic complexes — this is cationic). Option C reverses the order. Option D is gibberish. _Source: syllabus Chemistry Part 1, Ch 5, §5.3 (Nomenclature rules), p. 4–5._