The effective atomic number of Co(CO)₄ is 35 and hence is less stable. It attains stability by

both 2 and 3

The answer is (4) both 2 and 3.

The effective atomic number (EAN) of a metal complex is the number of electrons that the metal atom would have if it had the same number of valence electrons as the nearest noble gas. In the case of Co(CO)₄, the EAN is 35, which is one electron less than the EAN of the nearest noble gas, krypton.

The CO ligand is a strong field ligand, which means that it causes the electrons in the metal's d orbitals to pair up. This leaves one electron unpaired, which is why the EAN is 35.

In order to increase the EAN to 36, the metal atom can either reduce CO or dimerize.

Reduction of CO: If CO is reduced, it will donate an electron to the metal atom. This will fill up the one unpaired electron in the metal's d orbital, and the EAN will increase to 36.

Dimerization: If Co(CO)₄ dimerizes, two Co(CO)₄ molecules will combine to form a single molecule with a metal-metal bond. This will give the metal atom two more electrons, and the EAN will increase to 36.

Therefore, the correct answer is (4) both 2 and 3.