$[Fe(CN)_6]^{4-}$ and $[Fe(H_2O)_6]^{2+}$ show different colours in dilute solution because

• A
  $CN^{-}$ is a strong field ligand and $H_2O$ is a weak field ligand,hence the magnitude of $CFSE$ is different
• B
  both $CN^{-}$ and $H_2O$ absorb the same wavelength of energy
• C
  complexes of weak field ligands are generally colourless
• D
  the sizes of $CN^{-}$ and $H_2O$ are different,hence their colours are also different