Which of the following has longest $\mathrm{C}-\mathrm{O}$ bond length? (Free $\mathrm{C}-\mathrm{O}$ bond length in $\mathrm{CO}$ is $1.128 \mathrm{A}°$)

The greater the negative charge on the carbonyl complex, the more easy it would be for the metal to permit its electrons to participate in the back bonding, the higher would be the $\mathrm{M}-\mathrm{C}$ bond order and simultaneously there would be larger reduction in the $\mathrm{C}-\mathrm{O}$ bond order. 

Thus, ${\left[\mathrm{Fe}(\mathrm{CO}{)}_4\right]}^{2-}$has the lowest $\mathrm{C}-\mathrm{O}$ bond order means the longest bond length.