$A$ goes to a party hosted by his brother $B$. $B$ has a daughter $C$. $C$ has a brother $D$. What is $D$ to $A$?

Let's break down the family connections:
1. $B$ is the brother of $A$.
2. $C$ is the daughter of $B$.
3. $D$ is the brother of $C$, which means $D$ is the son of $B$.
4. Since $D$ is the son of $A$ 's brother, $D$ is the nephew of $A$.