Choose the option that best completes the sentence.
Justina used to be the best-behaved girl, ____?
Answer Details
In questions like this, the auxiliary verb used in the tag question should match the auxiliary verb or the verb used in the main sentence. All sentences have an auxiliary verb even though it is not visible. Sentences without visible auxiliary verbs carry the auxiliary verb "did" as in "She did used to be the best-behaved girl". Therefore, the correct tag question is "didn't she."