Concept:The tag question must use the correct auxiliary verb that matches the main sentence, especially when the main verb is "used to".
Explanation:"Used to" is a past habit expression, but it does not carry its own auxiliary verb in ordinary sentences.
When forming a tag question, the hidden auxiliary "did" is needed.
This is because "used to" behaves like a simple past verb.
So, you say: "She used to be the best-behaved girl, didn't she?"
The tag must be negative because the statement is positive.
Among the options, "didn't she" is the only one that correctly supplies the missing auxiliary "did".
Answer:D. didn't she