Hello, Apple! So nice to see you here again!
I find the formulation of both questions, (3) and (9), to be perfect. Let's see the difference which accounts for the presence of "the" in one case and its absence in the other:
(3) Which of the following is the best social scientific question?
(9) Which of the following is closest to the meaning of “Sartori's Ladder of Abstraction”?
Please note that it is the presence of the noun "question" that renders "the" necessary before the superlative in (3). In (9), the superlative is a predicative (non attributive) adjective, and that makes the article unnecessary.
We can find "the" before the superlative in the predicate when the noun is understood:
(a) Which of these girls is the most beautiful (girl)?
In (9), the tacit noun could be "statement," but notice that it doesn't even appear in the subject of the question:
(b) Which of the following is closest to the meaning of “Sartori's Ladder of Abstraction”?
If there is no noun in the subject, there is no need to include one in the predicate, and therefore no need to use the article before the superlative. If there is a noun in the subject, we can use a pronoun in the predicate and then "the" will be used. Notice, however, that a small change will need to be made for the sentence to sound idiomatic:
(c) Which of the following statements is the closest one in meaning to “Sartori's Ladder of Abstraction”?
Note: "mutiple" is a typo.