Solution

In phenol ($Ph-O-H$), the lone pair of electrons on oxygen is involved in resonance with the benzene ring, so oxygen carries a partial positive charge. Hence an incoming proton cannot attack it easily, and phenol is the most difficult to protonate.

Concept relative ease of protonation of oxygen compounds

Alcohols, Phenols and Ethers in past papers

65 questions from this chapter have appeared across 26 exam years.