SELECT 
    questions, 
    CONCAT(option_a, option_b, option_c, option_d) AS options, 
    COUNT(*) AS howmany
FROM 
    yourtable
GROUP BY
    questions, options
HAVING
    howmany > 1

##</code><code>##

ALTER IGNORE TABLE yourtable 
    ADD UNIQUE KEY (questions, option_a, option_b, option_c, option_d);