Step 1: Break the statement into pieces.
Arun says the woman is the daughter of his father's only son. We decode the inner phrase first.
Step 2: Decode 'my father's only son'.
Arun's father has only one son. That single son must be Arun himself, so 'my father's only son' equals Arun.
Step 3: Substitute back.
Replacing the phrase, the woman is the daughter of Arun.
Step 4: Name the relation.
The daughter of Arun is, plainly, Arun's daughter.
Step 5: Rule out other options.
She cannot be his sister, since that would need Arun's father, not Arun, to be her parent; mother and cousin contradict the direct daughter link.
Step 6: Conclude.
The woman is Arun's daughter, matching option 2.
\[ \boxed{\text{Daughter}} \]