Phenols are more acidic than alcohols, due to the stabilization of phenoxide ion via resonance. Ortho nitrophenol is most acidic of all because −NO2 is a electron withdrawing group. Thus, NO2 stabilise of negative charge on the oxygen of phenoxide ion. On the other hand, CH3 group is a electron donating group. Thus, acidic strength of phenol is decreased in cresol due to destabilisation of phenoxide ion. Thus, correct order is: 
Caution ⇒ Phenoxide ion is stabilized due to following resonating structures:



