Explain how the $-OH$ group attached to a carbon of a benzene ring activates it towards electrophilic substitution.

(N/A) The $-OH$ group is an electron-donating group due to the presence of lone pairs on the oxygen atom.
Through resonance,these lone pairs are delocalized into the benzene ring,which increases the electron density,particularly at the ortho and para positions.
This increased electron density makes the benzene ring more susceptible to attack by an electrophile,thereby activating the ring towards electrophilic substitution reactions.
The resonance structures of phenol are as follows:
(The resonance structures show the delocalization of the lone pair from the oxygen atom into the ring,creating negative charges at the ortho and para positions.)