Identify the species that acts as an acid.

According to the Brønsted-Lowry theory, an acid is a proton ($\text{H}^+$) donor. In the given reaction, $\text{NH}_4^+$ loses a proton to become $\text{NH}_3$.

$$\text{NH}_4^+ \text{(aq)} \longrightarrow \text{NH}_3 \text{(aq)} + \text{H}^+$$

A base is a proton ($\text{H}^+$) acceptor. In the reaction, $\text{OH}^-$ accepts a proton to become $\text{H}_2\text{O}$.

$$\text{OH}^- \text{(aq)} + \text{H}^+ \longrightarrow \text{H}_2\text{O (l)}$$

The acid in the reaction is $\text{NH}_4^+$. It acts as an acid because it donates a proton ($\text{H}^+$) to $\text{OH}^-$.

The acid is $\boxed{\text{NH}_4^+}$.
Reason: $\text{NH}_4^+$ donates a proton ($\text{H}^+$) to $\text{OH}^-$ to form $\text{NH}_3$ and $\text{H}_2\text{O}$.