Which of the following is most acidic?

I. Benzyl alcohol

II. Cyclohexanol 

III. Phenol

IV. m-Chlorophenol 

This is a multiple choice answer as classified in NCERT Exemplar

(IV) m-Chlorophenol 

Phenols are more acidic than sp³ and sp² hybridized alcohol.  The m-Chlorophenol is more acidic than phenol due to the -I effect (dominated by the + R effect) of chlorine atom increasing the acidity of phenol.