The correct answer is bold.
Bold is a font style that is used to emphasize text. It is often used to highlight important words or phrases, or to make text stand out from the rest of the document.
Italics is a font style that is used to indicate a secondary or subordinate idea. It is often used to introduce foreign words or phrases, or to show that a word is being used in a special or unusual way.
Underline is a font style that is used to indicate a link or URL. It is also sometimes used to emphasize text, but it is less common than bold or italics.
Regular is the default font style. It is not used to emphasize text, but it is used to display the majority of text in a document.