Which of the following is most acidic?

Phenols are more acidic than the benzyl alcohols and the cyclohexanol due to the contribution of the pi electrons of the oxygen in the aromatic pi system of phenol making phenoxide ion stable.

When an electron-withdrawing group like Cl is present in phenol structure its acidity increase more due to the due to the effective delocalization of negative charge in phenoxide ion.

These groups also weaken the O-H bond. Thus, H⁺ can be easily donated.

Thus, m-chlorophenol is more acidic than others.