The effective atomic number (EAN) equals the number of electrons remaining on the metal after forming its ion plus the electrons donated by all the ligands. In [Ni(CO)4] the nickel is in the zero oxidation state, so it retains all 28 of its electrons (atomic number of Ni is 28). Each carbon monoxide is a neutral two-electron donor, and four of them contribute 4 × 2 = 8 electrons. Adding these gives EAN = 28 + 8 = 36, which is the atomic number of the noble gas krypton, explaining the marked stability of tetracarbonylnickel(0). The answer 34 wrongly uses only three donor pairs, while 28 ignores the ligand contribution entirely. The value 32 mistakenly assumes nickel is in a +2 state. Sidgwick's EAN rule, presented in NCERT as a guide to carbonyl stability, predicts that complexes attaining a noble-gas count are especially stable. Plausibility check: the noble-gas configuration of 36 electrons matches the experimentally observed stability and volatility of Ni(CO)4 used in the Mond process.