Answer: Option D
Uncle, aunt, and brother are all members of the same family relationship category known as 'siblings and siblings-in-law'. Nephew is also a member of this category. Let's look at the definitions of each of these terms:

• Uncle: The brother of one's mother or father.
• Aunt: The sister of one's mother or father.
• Brother: A male sibling who shares the same parents.
• Nephew: The son of one's brother or sister, or the son of one's brother-in-law or sister-in-law.
• Boy: A male child or young adult.
• Child: A young human being below the age of puberty or below the legal age of majority.
• Adult: A person who has reached the age of majority and is considered to be fully grown or mature.

From the definitions above, we can see that the terms 'child', 'boy', and 'adult' are not in the same family relationship category as uncle, aunt, and brother. However, 'nephew' is a member of the same category, making it the correct answer.
Formulae:

• Sibling: A person who shares at least one parent with another person.
• Sibling-in-law: The sibling of one's spouse, or the spouse of one's sibling.
• Nephew: The son of one's brother or sister, or the son of one's brother-in-law or sister-in-law.

In summary, uncle, aunt, and brother are all members of the family relationship category 'siblings and siblings-in-law', and nephew is also a member of this category. Therefore, option D, Nephew, is the correct answer.