Among the following metal carbonyl, the $\mathrm{C}-\mathrm{O}$ bond order is lowest in:

$[\mathrm{V}{\left(\mathrm{CO}\right)}_6{]}^{-}$ has the lowest bond order.
$$\begin{gathered} \mathrm{V}=3{\mathrm{d}}^{3}4{\mathrm{s}}^2, \\ {\mathrm{V}}^{-}=3{\mathrm{d}}^{4}3{\mathrm{d}}^{4}4{\mathrm{s}}^2 \end{gathered}$$
Effective configuration $=3d^6\Rightarrow 3$ lone pair for back bonding with CO.
Three lone pairs for CO back bonding.
Thus, the correct option is B.