This Quiz focuses on 'these Questions of ours', as examples of expressing Demonstratives ('this/that' etc.) and Possession, in various combinations ~ so as to make sure you can pinpoint items and their owners, in order to avoid any potential embarrassment.
Answer 4 would have been acceptable had it said 'Which of his daughterS ... ' (i.e. which one amongst the two-or-more).
The accidental juxtaposition of ' ... his is it ... ' may look and sound strange, but in context is entirely correct; some writers would perhaps put a comma after 'it', to clarify the boundary between the component clauses in the sentence.