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 trace the family tree:
1. $A$ and $B$ are brothers.
2. $B$ has a daughter $C$.
3. $D$ is the brother of $C$, which means $D$ is the son of $B$.

Since $D$ is the son of $A$ 's brother ($B$), $D$ is the nephew of $A$.