Here is the code:
DECLARE @ParamVal int
SET @ParamVal = 2
SELECT
CASE @ParamVal
WHEN 0 THEN
SELECT MARKS,
CASE WHEN MARKS < 50 THEN 'FAIL'
END AS STATUS
WHEN 1 THEN
SELECT MARKS,
CASE WHEN MARKS >= 50 THEN 'PASS'
END AS STATUS
FROM TABLENAME
WHEN 2 THEN
SELECT MARKS,
CASE WHEN MARKS >= 50 THEN 'PASS'
ELSE 'FAIL'
END AS STATUS
FROM TABLENAME