When three hyphens, underscores, equal signs, asterisks, or number signs are typed and then the enter key is pressed, the Auto Format feature                     .

This question refers to an AutoFormat feature common in word processing software like Microsoft Word. When three hyphens (---), underscores (___), equal signs (===), asterisks (***), or number signs (###) are typed and followed by the Enter key, the software automatically converts them into a horizontal line, which functions as a border above a paragraph. Options B and D relate to list formatting, and C is for em dashes, which are distinct AutoFormat actions.