Quiz database design
Pretty good overall, only a few minor changes I would suggest:


quiz_id is redundant in the wp_quiz_participant_answers table, you
can get it via JOINs to either the wp_quiz_questions or
wp_quiz_participants tables
If you have any plans to allow something resembling a login or ability to keep taking quizzes as the same user you should probably break Users into their own table that wp_quiz_participants references. Kinda looks like you might be thinking along those lines with the quiz_participant_userid?


Performance is going to be based around your queries, pay close attention to the performance and add indexing when necessary.