A. Mark the text with an asterisk.
Explanation:
You can put an asterisk right at the point in the page where you feel the important information is while the other options are less efficient in that they do not help in retention of information or keeping a track of what was important.
You can mark a page as important but only if you believe the whole page is important not just a line, an asterisk can give specificity to the text which it would need in time of revision.