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Under Course Management in the lower left of your course site, click on the Grade Center link, then click on Full Grade Center. Find the column for the quiz in the Grade Center spreadsheet and match it to the row with the student who needs to retake the quiz. Where the column and row meet, click the double-down arrow button. From the drop-down menu, click View Grade Details. Click on the Clear Attempt button. When prompted, click OK to clear the attempt.
NOTE: If you enabled multiple attempts on a test or quiz, there are several options under Clear Attempts. You can select from: Last Attempt, First Attempt, Highest Scored Attempt, Lowest Scored Attempt, Specify Date, All Attempts