Which one is the most acidic compound?

Acidity is increased by electron-withdrawing groups (EWG) like $-{\text{NO}}_2$ and decreased by electron-donating groups (EDG) like $-{\text{CH}}_3$.$2,4,6$ -trinitrophenol (Picric acid) has three $-{\text{NO}}_2$ groups exerting very strong $-I$ and $-M$ effects, making it exceptionally acidic (more acidic than many carboxylic acids).$p$ -nitrophenol has one EWG.Phenol is neutral relative to these.$p$ -cresol has an EDG, making it the least acidic.