Add Emphasis to Text by Ruling Beneath It
Puzzle 17 of Spring Cleaning Pack
Question is: Add Emphasis to Text by Ruling Beneath It
and the answer is: UNDERLINE
Word Meaning:
UNDERLINE — A line placed underneath a piece of text in order to provide emphasis or to indicate that it should be viewed in italics or (in electronic documents) that it acts as a hyperlink.
Definitions from Wiktionary.