Answer
$NH_4^+$ is a weak acid.
Conjugate base: $NH_3$, which is a weak base.
Work Step by Step
$NH_4^+(aq) + H_2O(l) --> NH_3(aq) + H_3O^+(aq)$
As we can see in this reaction, $NH_3$ is the conjugate base of $NH_4^+$.
Since $NH_4^+$ is a weak acid, $N{H_3}^{}$ will be a weak base.