Answer to this is O-atom.
Explanation: The Bronsted acid-base theory is the backbone of chemistry. This theory focuses mainly on acids and bases acting as proton donors or proton acceptors.
where is the Lewis Acid and
is the Lewis Base and
is the Covalent Bond.
Reaction of dissociation of in
is given as:
In this reaction O-atom has lone pair in water and therefore it accepts the proton from forming a Lewis Base.