Acidity of the substituted phenols is affected by the natureof the substituents.
Presence of e− withdrawing group such as –NO2 increases the acidic strength whereas e− releasing or donating groups at o- and p- position decreases acidic strength of substituted phenol. In cresol (o- or p- cresol) methyl groups shows +I effect, soit destabilizes the anion formed after ionization hence cresol is less acidic and o-nitrophenol is more acidic than carbolic acid (phenol). Hence, the correct order for the acidic nature is o-cresol < phenol < o-nitrophenol.