Answer
$NH_4^+(aq)$ is an acid according to the Arrhenius definition.
Work Step by Step
According to the Arrhenius definition, an acid is a substance which produces $H^+$ ions in an aqueous solution.
As we can see from the below equation,
$NH_4^+(aq) \rightleftharpoons H^+(aq) + NH_3(aq)$
$NH_4^+$ produces $H^+$ ions in aqueous solutions. So it is an acid.
