Explanation:

- Hit the books is an informal way of saying to start studying seriously or with intensity.

- Option 1: To have a very long list doesn't relate to studying. It suggests having a lot of items to consider or tasks to complete.

- Option 2: To feel sorry about a mistake usually links to feeling regretful or apologetic, not related to studying.

- Option 3: Become very confused implies a state of bewilderment, rather than the focused effort associated with studying.

- Option 4: To study very hard captures the essence of "hit the books" perfectly, emphasizing dedicated effort in learning.