The correct statement(s) about the complexes [Ni(CO)4], [NiCl4]^2-, [Ni(CN)4]^2- and [Fe(CN)6]^4- is(are)

Count d electrons and ligand field strength. In $[\mathrm{Ni(CO)_4}]$ nickel is $\mathrm{Ni(0)}$, $d^{10}$, $sp^3$ and tetrahedral; all electrons are paired, so (A) is true. In $[\mathrm{NiCl_4}]^{2-}$, $\mathrm{Ni^{2+}}$ is $d^8$ and the weak-field chloride gives a tetrahedral geometry in which two electrons are unpaired, so $\mu=\sqrt{2\cdot4}=2.83$ BM; (B) is true. In $[\mathrm{Ni(CN)_4}]^{2-}$ the strong-field cyanide forces pairing of the $d^8$ ion and $dsp^2$ square planar geometry, diamagnetic; (C) is true. In $[\mathrm{Fe(CN)_6}]^{4-}$, $\mathrm{Fe^{2+}}$ is $d^6$ and cyanide gives the low-spin $t_{2g}^6$ configuration with no unpaired electrons, so it is diamagnetic. The value 4.9 BM would be the high-spin $d^6$ case, so (D) is false.