The correct answer is $\boxed{\text{C}}$.
In the given question, we are given the following definitions:
- $A * B$ means $A$ is the sister of $B$.
- $A $ B$ means $B$ is the mother of $A$.
- $A + B$ means $A$ is the brother of $B$.
- $A = B$ means $B$ is the father of $A$.
We are asked to find the option that means $M$ is the maternal uncle of $N$.
A maternal uncle is the brother of one’s mother. In other words, a maternal uncle is a man who is related to you through your mother.
Looking at the options, we can see that $\boxed{\text{C}}$ is the only option that satisfies this definition. In $\boxed{\text{C}}$, $N$ is the brother of $P$, and $P$ is the mother of $M$. Therefore, $M$ is the maternal uncle of $N$.
The other options do not satisfy the definition of a maternal uncle. In $\boxed{\text{A}}$, $M$ is the father of $N$, but $N$ is not the brother of $P$. In $\boxed{\text{B}}$, $N$ is the brother of $P$, but $P$ is not the mother of $M$. In $\boxed{\text{D}}$, none of the relationships are correct.