Answer:
Ca(OH)2 is the Arrhenius base
The Arrhenius theory of acids and bases defines a base as a substance that produces hydroxide ions (OH-) in aqueous solution.
Among the options given, Ca(OH)2 is the Arrhenius base because it dissociates in water to produce hydroxide ions.
HF, HCIO4, and CH4 are not Arrhenius bases as they do not produce hydroxide ions in aqueous solution. HF is an acid, HCIO4 is a strong acid, and CH4 is a nonpolar covalent compound.