$Sr(OH)_2(aq)$ 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, $Sr(OH)_2(aq) \rightarrow Sr^+(aq) + 2OH^-(aq)$ $Sr(OH)_2$ produces $OH^-$ ions in aqueous solutions. So it is a base.