Starting with Radha's statement: "This person is the son of my husband's father's only daughter."
Breaking down the relationship step by step:
"My husband's father" = Radha's father-in-law
"My husband's father's only daughter" = The only daughter of Radha's father-in-law
Since Radha's father-in-law has only one daughter, this daughter is Radha's husband's sister (Radha's sister-in-law).
"The son of my husband's father's only daughter" = The son of Radha's sister-in-law
The son of Radha's husband's sister is Radha's nephew.
Therefore, the person in the photograph is Radha's nephew.
The correct answer is Option 4: Nephew.