Co(CO)4 is a compound in which cobalt is coordinated with four carbon monoxide ligands. The EAN (Effective Atomic Number) rule is a guideline used to predict the number of electrons that a transition metal should have in its complex to achieve maximum stability. According to the EAN rule, the sum of the metal's valence electrons and the electrons donated by the ligands should be equal to the atomic number of the next noble gas.

1. Oxidizing Character:
Oxidizing character refers to the ability of a compound to accept electrons or cause other substances to lose electrons. In the case of Co(CO)4, each carbon monoxide ligand donates two electrons to the cobalt atom. Since carbon monoxide is a weak pi donor ligand, it does not significantly affect the oxidation state of the metal. Therefore, the oxidizing character of Co(CO)4 is not the reason it follows the EAN rule.

2. Reduction:
Reduction refers to the gain of electrons or the ability to donate electrons to other substances. In Co(CO)4, the cobalt atom has an oxidation state of zero, and each carbon monoxide ligand donates two electrons to the metal. As a result, the cobalt atom gains a total of eight electrons from the ligands. This reduction of the cobalt atom to achieve a stable electron configuration is a part of the reason why Co(CO)4 follows the EAN rule.

3. Dimerization:
Dimerization refers to the formation of dimers, which are compounds consisting of two identical molecules or ions bonded together. Co(CO)4 does not dimerize, as it is a monomeric compound with a tetrahedral structure. Therefore, dimerization is not the reason why Co(CO)4 follows the EAN rule.

4. All of these:
The correct answer is option 'D' because all of the given options are related to the EAN rule. Co(CO)4 follows the EAN rule because the cobalt atom gains electrons through reduction by the carbon monoxide ligands. This results in a total electron count that is equal to the atomic number of the next noble gas, which is the goal of the EAN rule.