Which one of the following compounds is most acidic?

Phenols are more acidic than alcohols, due to the stabilization of phenoxide ion via resonance. Ortho nitrophenol is most acidic of all because $-\mathrm{NO}_2$ is a electron withdrawing group. Thus, $\mathrm{NO}_2$ stabilise of negative charge on the oxygen of phenoxide ion. On the other hand, $\mathrm{CH}_3$ 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 $\Rightarrow$ Phenoxide ion is stabilized due to following resonating structures: