This looks suspiciously like a class assignment. If you had devised this structure yourself, you should know the answer because you would have built the tables yourself. Anyone who can build that complex a layout would be experienced enough to build the right kind of form.
Just to not blow you off completely, though, it is obvious you need a form that would let you select a particular exam, a subject, and a specific student. If you have those items selected, then one more text box would be needed for the grade itself.
I can't reasonably send you a solution, I can only suggest that you use the form builder with control wizards enabled. This is a "Help" forum, not a "Do my work for me" forum.
In your application, you asked for data entry so what I might do is build a bound form based on your Marks_Entry table. On it put a combo box to select the student. Put another combo box to select the subject. Put yet another combo box to select a specific exam. Finally, one more combo for the Class ID. Then put a text box on the form to allow free-form grade entry. You can use the "standard" Access control bar at the left side of the form that allows you to choose when to save a new record. That would be my idea of the simplest approach.
You need to fix your table relationships before anything. There should only be one way to trace a path between 2 tables. Between tbl_students and tbl_Classes I can trace 2 paths--one thru Class_Enroll and one through tbl_Marks_Entry. That is wrong. I think a lot of tables are related incorrectly.
tbl_students should be related to Class_Enroll
tbl_Classes should be related to Class_Enroll
tbl_Subjects should be related to tbl_Subjects
You should have a new table called tbl_Exams which is related to Class_Enroll
tbl_Exam_Type should be related to tbl_Exams
tbl_Marks_Entry should be related to tbl_Exams
This wil require adding/removing appropriate foreign keys from the new relationships in each table.