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

Ortho-nitrophenol has intramolecular hydrogen bonding (chelation), which makes it more volatile. Para-nitrophenol has intermolecular hydrogen bonding, making it less volatile and giving it a higher boiling point. Due to this significant difference in volatility, they can be separated by steam distillation; the ortho isomer distills over with steam.