C. They highlight and signpost key points of the text.
Explanation:
In a story or an article all the components that are not the main body of text are what is known as text features, they help to highlight specific parts of the text like placing a table of contents, the index, or a glossary of terms; but also some features within the text like headings, bold words and the usage of sidebars. On the graphic side we can see as examples of text features pictures, captions and diagrams all of those help to signpost key points of the text.
