The correct answer is C. Modifying.
To modify something means to change it in some way. In the context of a document, this could mean making changes to the text, formatting, or layout. It could also mean adding or removing content.
Creating a document means making it from scratch. This could involve typing out the text, adding images, or creating a table.
Editing a document means making changes to the existing content. This could involve correcting errors, adding or removing information, or changing the order of the content.
Adjusting a document means making minor changes to the formatting or layout. This could involve changing the font size, font color, or margins.
None of the other options are correct because they do not accurately describe the process of changing an existing document.