Complex compounds are substances that consist of a central metal ion or atom surrounded by ligands. Ligands are molecules or ions that donate a pair of electrons to the metal ion, forming coordinate bonds. The oxidation number of a metal in a complex compound is the charge that the metal would have if all the ligands were removed along with their associated electrons.

In the given options, the complex compound [Ni(CO)4] stands out because it contains a metal ion with an oxidation number of zero. Let's break down the compound and analyze its structure and oxidation state.

1. [Ni(CO)4]:
- The metal ion in this complex compound is nickel (Ni).
- The ligands are four carbon monoxide (CO) molecules.
- Carbon monoxide is a strong π-donor ligand, meaning it donates its electron pair to the metal.
- Each carbon monoxide molecule forms a coordinate bond with the nickel ion, resulting in a square planar geometry.
- The oxidation number of carbon in carbon monoxide is +2, and there are four carbon monoxide ligands. Therefore, the total oxidation number contributed by the ligands is +8.
- Since the overall charge of the complex is zero, the oxidation number of the nickel ion must be -8 to balance out the ligand contributions.
- Thus, the oxidation number of nickel in [Ni(CO)4] is zero.

Therefore, option C, [Ni(CO)4], is the correct answer because it represents a complex compound in which the oxidation number of the metal is zero.