Which of the following complex will show largest splitting of d-orbitals?

The magnitude of $$d$$-orbital splitting depends primarily on the strength of the ligands according to the Spectrochemical Series. 

For a constant metal ion in the same oxidation state ($$Fe^{3+}$$ in all these cases), the ligand strength determines the extent of splitting.

Ligand Strength Order: 

$$CN^->NH_3>C_2O_4^{2-}>F^-$$

[Fe(CN)$$_6$$]$$^{3-}$$: Cyanide $$(CN^{-})$$ is a strong-field ligand. 

It causes the maximum repulsion with the $$d$$-orbitals of the metal, leading to the largest crystal field splitting energy $$\triangle_o$$.

Other options: $$NH_{3}$$ is a moderate ligand, while $$C_{2}O_{4}^{2-}$$ and $$F^{-}$$ are weak-field ligands, resulting in smaller splitting.

The complex that will show the largest splitting of $$d$$-orbitals $$\triangle_o$$ is C: $$[Fe(CN)_6]^{3-}$$.