The original sentence is in the affirmative form, "Few students knew the solution." To create a question tag, we typically use the auxiliary verb from the main sentence and invert the subject and auxiliary verb. In this case, the auxiliary verb is "did," so the question tag should be "Did they?"

The other options don't follow the correct structure for forming question tags. "Were not they?" and "Would not they?" don't match the structure of the original sentence. "Do not they?" is not appropriate here because it doesn't use the past tense auxiliary verb, and it should be "Did they?" to maintain tense agreement.