The correct order of decreasing acidity of nitrophenols will be:

The acidity of nitrophenols depends on the ability of the nitro group to stabilize the phenoxide ion through resonance and inductive effects. The para isomer exhibits maximum resonance stabilization due to direct conjugation, making it the strongest acid. The ortho isomer has intramolecular hydrogen bonding that stabilizes the neutral molecule, reducing acidity relative to para, but still exhibits resonance and inductive withdrawal, making it more acidic than the meta isomer, which only has inductive withdrawal. Thus, the decreasing order of acidity is p-nitrophenol > o-nitrophenol > m-nitrophenol.