Student who got MAX marks in subject
You can use CTE with RANK() function. Read more here: 

Example:

WITH ordermarks AS (
    SELECT [STUD_ID],[SUBJECT],[MARKS], RANK() OVER (PARTITION BY [SUBJECT] ORDER BY [MARKS] DESC) AS rn
    FROM marks
)
SELECT [STUD_NAME],[SUBJECT],[MARKS]
FROM ordermarks om
JOIN Students s ON s.[STUD_ID]=om.[STUD_ID] AND rn=1