उत्तर : 3. "didn't she"
व्याख्या :

In English grammar, when using negative statements (like "did not"), the question tag that follows should be positive. Therefore, the correct question tag for "She did not know the answer" should be "Did she?" instead of "Didn't she?"

So, the corrected sentence would be: "She did not know the answer, did she?"

Correct answer: c) didn't she