Correct option (d) steam distillation.
Explanation:
Steam distillation is employed as there is a large difference between boiling points of ortho and para-nitrophenols. This is due to the presence of intramolecular H-bonding in the case of o-nitrophenol (so low b.pt.) and intermolecular H-bonding in p-nitrophenol (so high b.pt.).