This is because of buggy questions imported by file import.
1- Run below query and then save the result in text file
SELECT t1.id, t1.name
FROM mdl_question t1
LEFT JOIN mdl_qtype_multichoice_options t2 ON t1.id = t2.questionid
WHERE t1.qtype=’multichoice’ AND t2.id IS NULL
2- Then run another query to delete buggy questions by ID
Delete from mdl_question where id in (id1, id2, ….)