The complex exhibits the lowest energy absorption band is:

The complex that exhibits the lowest energy absorption band is [NiCl₄]^2–. This is because Cl^– is a weak field ligand, which means that it does not split the d orbitals of the Ni²⁺ ion very much. This results in a low crystal field splitting energy (Δ_o), which corresponds to a low energy absorption band.

The other complexes in the answer choices have stronger field ligands, which results in a higher Δ_o and a higher energy absorption band.

So the answer is (a) [NiCl₄]^2–.