Explanation

(a)
Oxidation state of Ni in $Ni(CO)_{4}$ is zero.
3$d$ 4$s$ $4p$
[table] ⥮ | ⥮ | ⥮ | $\upharpoonleft$ | $\upharpoonleft$ | | ⥮ | | | | [/table]
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CO is a strong ligand. It causes pairing of electrons. Hence, there is no unpaired electrons in $Ni(CO)_{4}$.