What does it mean when a question number is circled in a textbook

This is only my opinion.  I could be wrong.

My guess is:

Somebody else used that book before you, maybe last year or
the year before, and it was somebody who didn't mind writing
in his book.

One day he didn't have time to write down the homework in his
assignment notebook, so he just circled the homework problems
in his textbook.