The compound that is most difficult to protonate is :

Question : The compound that is most difficult to protonate is :

answer : from all above options the correct answer is option “4” , first of all you must know the definition of protonation ? , when we need to form conjugate acid we need to add proton to that atom or molecule and adding a proton to it process is called protonation , here you must remember that proton means we are adding hydron, or hydrogen cation that is why this process also known as hydronation when hydrogen to be added.

answer explanation : Protonation will be most difficult in case of phenol as lone pair of electrons on oxygen atom gets involved in the resonance with the benzene ring. it means in phenol structure shown in option 4 is hard to add proton to it because there is resonance take place on benzene ring.
that is why we have marked option (4) for correct Answer.