Introducing a man to her husband, a woman said, 'This man's father is the only son of my grandfather.' How is the woman related to this man?

Introduction / Context:
This blood relation question is about a woman introducing a man to her husband. She describes the man's father as the only son of her grandfather. You must deduce how she is related to the man. The key point is that the only son of my grandfather is normally my father, so if the man's father is that person, then the man is my brother.

Given Data / Assumptions:
- Speaker: a woman. - She is talking about a man and introducing him to her husband. - She says: This man's father is the only son of my grandfather. - Only son of my grandfather is interpreted as her father in the usual exam convention. - Standard family structure is assumed.

Concept / Approach:
First identify who my grandfather's only son is. If a woman speaks of her grandfather and says he has only one son, that son must be her father (because she is his granddaughter). The man's father is this only son, meaning the man's father and the woman's father are the same person. Two people who share the same father (and usually also the same mother in these puzzles) are siblings. Since the speaker is a woman, she is the man's sister.

Step-by-Step Solution:
Step 1: Let G be the woman's grandfather. G's only son is mentioned in the statement. Step 2: Because G has only one son and the speaker is his granddaughter, that son must be her father. There is no other male child for G. Step 3: The statement says that this man's father is the only son of my grandfather. Therefore, the man's father is that same person, the woman's father. Step 4: If two people have the same father, they are siblings in the basic puzzle model. Step 5: The speaker is a woman, so she is the man's sister, and the man is her brother.

Verification / Alternative check:
Draw a simple diagram: put the grandfather at the top. He has one son F. F has at least two children: the woman and the man she is introducing. Both children are in the same generation and share the same father F. In such a diagram, the relation between the woman and the man is always brother–sister. Since the question asks how the woman is related to the man, the correct term is sister.

Why Other Options Are Wrong:
- Mother: The woman shares a father with the man, so she is not in the generation above him. - Aunt: An aunt would be a sister of the man's father, but here she is a child of that father, not his sibling. - Daughter: For her to be his daughter, the man would need to be her father, which contradicts the generational structure. - Cousin: Cousins share grandparents but have different parents; here, they share the same father, not just grandparents.

Common Pitfalls:
A typical mistake is to misinterpret only son of my grandfather as a generic uncle rather than the father. However, since the speaker is a granddaughter and no other children of the grandfather are mentioned, the most direct and intended reading is that the only son is her father. Another pitfall is to focus on the phrase to her husband and wonder if the man could be related by marriage, but that clause is just narrative and does not change the blood relationship being described.

Final Answer:
The woman is the man's sister.