Database design - many to many
Considering that the question is labeled database-design I will offer a solution without that CSV field. Even if you decide to keep it, this may offer some ideas.


Question contains all possible questions.
Answer has all possible answers (to any question)
QuestionAnswer contains all QA combinations offered, note that there must exist a default answer for "I do not know".
UserQA contains questions user answered.
Note that rule "There can be one and only one correct answer for each question" (IsCorrect = 'Y'), has to be implemented on the application level or as a stored procedure/trigger.




To get questions that a user with specific_UserId has not yet answered

select  qq.QuestionID
from Question as qq
where not exists (select 1 
                    from UserQA as xx 
                   where xx.QuestionID = qq.QuestionID
                     and xx.UserID     = specific_UserId );


Or, better if there may be some new questions which do not have defined answers yet

select distinct qq.QuestionID
from QuestionAnswer as qq
where not exists (select 1 
                    from UserQA as xx 
                   where xx.QuestionID = qq.QuestionID
                     and xx.UserID     = specific_UserId );