The C-O bond length is the shortest in

Options

1. A$\left[\operatorname{Cr}(\mathrm{CO})_6\right]$
2. B$\left[\mathrm{Mo}(\mathrm{CO})_6\right]$
3. C\left[\mathrm{Mn}(\mathrm{CO})_6\right]^{+}
4. D\left[\mathrm{V}(\mathrm{CO})_6\right]^{-}