Assuming these are SQL tables, and you are trying to do it in SQL:
SELECT sm.MarksId, sm.StudentId, g.Grade AS Grade FROM StudentMarks sm
JOIN Grade g ON sm.Marks <= g.MaxMark AND sm.Marks >= g.MinMark
[edit]
Or better:
SELECT sm.MarksId, sm.StudentId, g.Grade AS Grade FROM StudentMarks sm
JOIN Grade g ON sm.Marks BETWEEN g.MinMark AND g.MaxMark
[/edit]