(A) Mn⁺=3d⁵4s¹;in presence of CO, effective configuration=3d⁶4s⁰.
Three lone pair for back bonding with vacant orbital of C in CO.
(B) $\mathrm{F}{\mathrm{e}}^0=3{\mathrm{d}}^{6}4{\mathrm{s}}^2$ ;in presence of CO, effective configuration $=3{\mathrm{d}}^8$ .
Four lone pair for back bonding with CO.
(C) $\mathrm{C}{\mathrm{r}}^0=3{\mathrm{d}}^{5}4{\mathrm{s}}^1$ ;effective configuration $=3{\mathrm{d}}^6$ .
Three lone pair for back bonding with CO.
(D) ${\mathrm{V}}^{-}=3{\mathrm{d}}^{4}4{\mathrm{s}}^2$ ;effective configuration $=3{\mathrm{d}}^6$ .
Three lone pair for back bonding with CO.
Maximum back bonding is present in $\mathrm{F}\mathrm{e}(\mathrm{C}\mathrm{O}{)}_5$ ​therefore, CO bond order is lowest here.