The correct answer is C. Aunt.
The girl’s father has only one son. The lady is the daughter-in-law of the grandmother of the girl’s father’s only son. This means that the lady is married to the grandmother of the girl’s father. The grandmother of the girl’s father is the girl’s great-grandmother. This means that the lady is married to the girl’s great-uncle. The great-uncle of the girl is the brother of the girl’s grandfather. This means that the lady is the aunt of the girl.
The other options are incorrect because they do not accurately describe the relationship between the lady and the girl.
- Sister-in-law is incorrect because the lady is not married to the girl’s brother.
- Mother is incorrect because the lady is not the girl’s biological mother.
- Mother-in-law is incorrect because the lady is not married to the girl’s husband.
- Cousin is incorrect because the lady is not a descendant of the girl’s parents.