A. You can change the answer key, convert it to the formula file (or just modify the formula file) and regrade the test.
This will replace the original answers and grades file with a new one (you may want to save your original file first). The two files will be the same except for the changed response columns (i.e., 'correct' or 'incorrect') and the new scores.
If the answer to question 5 was 'A', this would be the original line in the formula file:
1 test005 correct incorrect, the answer is A field005=="A"
If you wish to change the correct answer to 'C', this is the new line in the formula file:
1 test005 correct incorrect, the answer is C field005=="C"
If you want both answers 'A' and 'C' to be counted as correct, change the line to:
1 test005 correct incorrect, the answer is A or C (field005=="A") OR (field005=="C")
To regrade the test after changing the formula file, you must go to this URL:
(Of course, you must replace "yourServer", "yourSite", "yourCategory" and "yourTest" with the names appropriate to your situation).