: Use the Type Tool (T) to highlight specific words, or the Selection Tool (V) to select an entire text box.
: If you don't see the underline icon, click the "hamburger" menu (three horizontal lines) in the corner of the panel and select Show Options . underline in illustrator
This article covers three methods: the (quick but limited), the Custom Thick Underline (professional), and the Zigzag/Wavy Underline (creative). : Use the Type Tool (T) to highlight