The compound that is most difficult to protonate is:
Protonation of phenol is difficult.
Reason:- In alcohols the electron density on oxygen is high.
due to localisation of its lone pair and e- donating nature of alkyl group. So, their protonation is easy.
However, in phenols the lone pair of oxygen is in resonance with the benzene ring and thus is delocalised.
So, protonation of phenol is difficult.