Type three hyphens and press Enter: `—`
Type three underscores and press Enter: `___`
Type three asterisks and press Enter: `***`
Type three equal signs and press Enter: `===`
Type three tildes and press Enter: `~~~`
Use Home > Borders > Horizontal Line
Use Insert > Shapes > Line
Use Insert > Table > 1×1 table and adjust the border
Use the keyboard shortcut for a border line by typing the characters above and pressing Enter