Which one of the following is square planar in structure and has diamagnetic property ?

In [Ni(H₂O)₆]²⁺, the oxidation state of Ni is +2 and its outer electronic configuration becomes 3d⁸ 4s⁰. Electrons remain unpaired in 3d-subshell of Ni because water acts as a weak ligand.

In [Ni(CO)₄], oxidation state of Ni is 0 and when CO (strong ligand) approaches Ni, then all electrons from 4s subshell also gets paired in 3d. As a result, complex becomes diamagnetic but it forms a tetrahedral complex.

In [Ni(CN)₄]^2-, the oxidation state of Ni is +2 and electrons of 3d-subshell gets paired when cyanide ion (strong ligand) approaches. The hybridization of the complex becomes dsp², has square planar geometry and is diamagnetic.

In [NiCl₄]^2-, oxidation state of Ni is +2 and the electrons in 3d-subshell remain unpaired because chloride ion is a weak field ligand.