The diamagnetic species is:

2-]$[\text{Ni}(\text{CN}{)}_4{]}^{2-}$: ${\text{Ni}}^{2+}$ ($d^8$ configuration) with ${\text{CN}}^{-}$ (strong field ligand) forms a square planar complex ($ds{p}^2$ hybridization). All 8 electrons are paired, making it diamagnetic.$[{\text{NiCl}}_4{]}^{2-}$: ${\text{Cl}}^{-}$ (weak field) forms a tetrahedral complex with 2 unpaired electrons (paramagnetic).$[{\text{CoCl}}_4{]}^{2-}$ and $[{\text{CoF}}_6{]}^{3-}$: Both have weak field ligands and unpaired electrons (paramagnetic).