Answer is: hydroxide.
According to Bronsted-Lowry theory acid are donor of protons and bases are acceptors of protons (the hydrogen cation or H⁺).
The hydroxide anion (OH⁻) is Bronsted base and it can accept proton and become conjugate acid H₂O.
Balance chemical reaction with acid:
OH⁻(aq) + H⁺(aq) ⇄ H₂O(l).