What you are going to want to do is numerically name each radio button set, which coincides with the row from the database. Something like 'radioset1', 'radioset2', ...
You can then loop over the results while $_POST['radioset'.$i] is available (start $i at zero, and increment it inside the while loop). If that value matches the current cat_letter from the database, then increment your 'score' counter.
Let me know if you need more explanation, its late and I'm tired, so probably aren't explaining it well.
Actually, you can read up on it some here:
http://php.codeislogic.com/database-...-radio-buttons