The complex which exhibits lowest energy electronic absorption band is [NiCl4]2. This can be explained through the following points:

Explanation:

1. Determining factors for energy of electronic absorption band
The energy of electronic absorption bands depends on various factors such as the nature of the ligand, oxidation state of the metal ion, coordination number, geometry of the complex, and the extent of d-orbital splitting.

2. Comparison of given complexes
Out of the given complexes, [NiCl4]2, [Ni(H2O)6]2, [Ni(CN)4]2, and Ni(CO)4, the complex with the lowest energy of electronic absorption band is [NiCl4]2.

3. Reason for lowest energy in [NiCl4]2
The reason for this can be attributed to the nature of the ligand. Chloride ion being a weak field ligand, it causes less splitting of the d-orbitals of the nickel ion. This results in a lower energy of electronic absorption band as compared to the other complexes where ligands such as water, cyanide, and carbon monoxide cause a greater extent of d-orbital splitting due to their strong field nature.

4. Other factors affecting energy of electronic absorption band
Although the oxidation state of the metal ion, coordination number, and geometry of the complex also affect the energy of electronic absorption bands, in this case, the nature of the ligand is the primary determining factor.

Conclusion:
Thus, [NiCl4]2 is the complex that exhibits the lowest energy of electronic absorption band due to the weak field nature of the chloride ligand causing less splitting of the d-orbitals of the nickel ion.