Which textbook feature identifies and divides the text into sections? exercises overviews graphics headings?

The right option is; headings

Headings is the textbook feature that identifies and divides the text into sections.  

Headings are phrases or words that are usually written at the top of passages or pages in a book to explain what the succeeding section is all about. Headings identify and divide a book into sections and it can be considered as the topic of that particular section. Headings are usually bigger than subheadings and it helps readers to identify the most important points of each section in a book.  

a. to encourage apoptosis