Answer is: hydroxide ions (OH⁻).
An Arrhenius base is
a substance that dissociates in water to form hydroxide ions (OH⁻).
For example sodium hydroxide:
NaOH(aq) → Na⁺(aq)
+ OH⁻(aq) or barium hydroxide: Ba(OH)₂(aq) → Ba²⁺(aq) + 2OH⁻(aq).
According to this theory
acids are substances which produce hydrogen ions in water solution.
