Final answer:
Al(OH)₃ is a base according to the Arrhenius definition because it contains OH- ions.
Step-by-step explanation:
The Arrhenius definition of an acid is a substance that increases the amount of H+ in an aqueous solution. The Arrhenius definition of a base is a substance that increases the amount of OH- in an aqueous solution. Based on this definition, we can identify Al(OH)3 as a base because it contains OH- ions.