(D) The coordination number of $Ni$ in $[Ni(C_2O_4)_3]^{4-}$ is $6$ because $C_2O_4^{2-}$ (oxalate) is a bidentate ligand and there are $3$ such ligan

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(D) The coordination number of $Ni$ in $[Ni(C_2O_4)_3]^{4-}$ is $6$ because $C_2O_4^{2-}$ (oxalate) is a bidentate ligand and there are $3$ such ligands,so $2 \times 3 = 6$. Let the oxidation state of $Ni$ be $x$. $x + 3(-2) = -4$ $x - 6 = -4$ $x = +2$ Sum of coordination number and oxidation number = $6 + 2 = 8$.