The correct answer is C. Maternal uncle.
The woman’s father has only one daughter, who is married to the man. This means that the man is the husband of the woman’s only sister. The woman’s sister is the woman’s maternal aunt, and the man is therefore the woman’s maternal uncle.
Option A is incorrect because the man is not the woman’s brother. The woman’s brother would be the son of the woman’s mother and father. The man is not the son of the woman’s mother and father.
Option B is incorrect because the man is not the woman’s father-in-law. The woman’s father-in-law would be the father of the woman’s husband. The man is not the father of the woman’s husband.
Option D is incorrect because the man is not the woman’s husband. The woman’s husband would be the man who is married to the woman. The man is not married to the woman.
Option E is incorrect because the man is related to the woman. The man is the husband of the woman’s only sister. This means that the man is the woman’s maternal uncle.