Это должно сделать это ...
SELECT QuesNo, SUBSTRING(Answers, 1, LENGTH(Answers) - 1) AS Answers
FROM (
SELECT QuesNo,
CASE
WHEN A <> 0 THEN 'A,'
ELSE ''
END +
CASE
WHEN B <> 0 THEN 'B,'
ELSE ''
END +
CASE
WHEN C <> 0 THEN 'C,'
ELSE ''
END +
CASE
WHEN D <> 0 THEN 'D,'
ELSE ''
END AS Answers
FROM yourtable
) Foo