The oxidation number of cobalt in $K[Co(CO)_4]$ is
$(i)$ $+1$ $(ii)$ $+3$ $(iii)$ $-1$ $(iv)$ $-3$

(C) We know that $CO$ is a neutral ligand and $K$ carries a charge of $+1$.
Therefore,the complex can be written as $K^{+}[Co(CO)_4]^{-}$.
Let the oxidation number of $Co$ be $x$.
For the complex ion $[Co(CO)_4]^{-}$,the sum of oxidation numbers is equal to the charge on the ion:
$x + 4(0) = -1$
$x = -1$
Therefore,the oxidation number of $Co$ in the given complex is $-1$. Hence,option $(iii)$ is correct.