The most suitable method of separation of $1:1$ mixture of ortho and para-nitrophenols is:

[Image showing intramolecular vs intermolecular hydrogen bonding in nitrophenols]Ortho-nitrophenol has intramolecular hydrogen bonding, making it more volatile. Para-nitrophenol has intermolecular hydrogen bonding, giving it a higher boiling point. Due to this significant difference in volatility, they are easily separated by steam distillation.