Which of the following substituted phenols is the strongest acid?

Correct option is C

Ans.(c)
Acidic strength of phenol is enhanced by the presence of electron- withdrawing groups which stabilized the phenoxide ion.
While presence of electron- donating group like CH₃,C₂H₅ destabilizes the phenoxide ion, thus decreasing the acidic strength.

Due to the presence of strong electron- withdrawing group–NO₂(–I, –R), p-nitro phenol is the strongest acid.