Among the following metal carbonyls, the C-O bond order is lowest in

(a) $\mathrm{Mn}^{+}=3 d^5 4 s^1$ In presence of $\mathrm{CO}$ effective configuration $=3 d^6 4 s^0, 3$ lone pair for back bonding with vacant orbital of $\mathrm{C}$ in $\mathrm{CO}$. (b) $\mathrm{Fe}^{\circ}=3 d^6 4 s^2$ in presence of $\mathrm{CO}$ effective configuration $=3 d^8, 4$ lone pair for back bonding with $\mathrm{CO}$. (c) $\mathrm{Cr}^{\circ}=3 d^5 4 s^1$, effective configuration $=3 d^6$ 3 lone pair for back bonding with $\mathrm{CO}$. (d) $\mathrm{V}^{-}=3 d^4 s^2$, effective configuration $=3 d^6, 3$ lone pair for back bonding with $\mathrm{CO}$. Maximum back bonding in $\mathrm{Fe}(\mathrm{CO})_5$, therefore $\mathrm{CO}$ bond order is lowest here.