(a) o-Nitrophenol involves an intramolecular hydrogen bond whereas p-nitrophenol involves an intermolecular hydrogen bond. Consequently, the former exists as a single molecule whereas the latter exists as a multimolecular form.

 (d) Because of intramolecular hydrogen bonding in o-nitrophenol, the latter has a lower boiling point as compared to p-nitrophenol (which has intermolecular hydrogen bonding).