The correct spelling is 'mischievous' with 'ie' after 'm' and 'ou' before 'us'. Option A incorrectly uses 'ei' and 'iv', while C uses 'ious' instead of 'ous', and D compounds both errors with 'ei' and 'iv'. The root comes from 'mischief', and the adjective form maintains the 'ie' vowel order.