Answer
$KOH(aq)$ is a base according to the Arrhenius definition.
Work Step by Step
According to the Arrhenius definition, a base is a substance which produces $OH^-$ ions in an aqueous solution.
As we can see from the below equation,
$KOH(aq) \rightarrow K^+(aq) + OH^-(aq)$
$KOH$ produces $OH^-$ ions in aqueous solutions. So it is a base.
