The compound that is an Arrhenius base is Ca(OH)₂ (calcium hydroxide).
Arrhenius bases are defined as substances that increase the concentration of hydroxide ions (OH⁻) in aqueous solution. Calcium hydroxide dissociates in water to produce Ca²⁺ and OH⁻ ions, thus qualifying as an Arrhenius base.
The other compounds listed are not Arrhenius bases:
- HNO₃ (nitric acid) is an Arrhenius acid.
- H₃PO₄ (phosphoric acid) is also an Arrhenius acid.
- HClO (hypochlorous acid) is likewise an Arrhenius acid.