Answer
$NH_3$ is a weak base.
Conjugate acid: ${NH_4}^+$, which is a weak acid.
Work Step by Step
$NH_3(aq) + H_2O(l) --> {NH_4}^+(aq) + OH^-(aq)$
As we can observe, ${NH_4}^+$ is the conjugate acid of $NH_3$.
Since $NH3$ is a weak base, his conjugate acid will be weak.