Looking at a portrait, Hanni said "His father is husband of my father's only daughter".

Hanni's father's only daughter is Hanni herself (or Hanni's sister if Hanni is male, but context suggests Hanni is female). The man in the portrait's father is the husband of Hanni's father's only daughter—meaning the man's father is Hanni's husband. Therefore, the man in the portrait is Hanni's son.

Step-by-step Derivation:
Step 1: Identify 'my father's only daughter' = Hanni (assuming Hanni is female) or Hanni's sister. Step 2: 'husband of my father's only daughter' = Hanni's husband. Step 3: 'His father' (the portrait man's father) = Hanni's husband. Step 4: Therefore, the man in the portrait is Hanni's son. Correct answer: D) Son.