$\left(A\right)$ $\left[Fe\left(C{O}_5\right)\right]\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{ }\left(Ni{\left(CO\right)}_4\right)$ Complexes have 18- electrons in their valence shell
$\left(B\right)$ Carbonyl complexes are predominantly low spin complexes due to strong ligand field
$\left(C\right)$ As electron density increases on metals (with lowering oxidation state on metals), the extent of synergic bonding increases. Hence M - C bond strength increases
$\left(D\right)$ While positive charge on metals increases and the extent of synergic bond decreases and hence C - O bond becomes stronger.