Step 1: Identifying the connection.
The man's father is the sibling of his mother's brother. Therefore, the man's father and maternal uncle are siblings. Consequently, the man's mother's brother is his maternal uncle.
Step 2: Defining the relationship.
Because the man's father and the man’s maternal uncle are siblings, the man is the nephew of his mother’s brother.
Step 3: Answer.
The correct choice is (A) Nephew.