Which one of the following compounds is most acidic?

Step-by-step solution

Phenols are much more acidic than alcohols, due to the stabilisation of phenoxide ion by resonance. Phenoxide ion is stabilized due to following resonating structures : While in alcohols Ortho nitrophenol is most acidic because in it - NO ₂ electron attracting group is attached on ortho position which helps in stabilising of negative charge on the oxygen of phenoxide ion. Hence, due to this reason acidic character of phenol is increased, while on attachment of - CH ₃ group (electron donating group) acidic strength