Which has largest C-O bond length in the given, complexes?

Here, among the given complexes, $\left[\mathrm{Fe}\right(\mathrm{CO}{)}_4{]}^{2-}$ has the longest C-O bond length. The bond length of the C-O bond increases as length the negative charge on the metal carbonyl complex increases, while the bond of the metal-carbon bond decreases.
The correct answer is B.