When PowerPoint has many choices for something, such as styles for a shape or types of WordArt or animation effects, a few choices show on the Ribbon. To see the full gallery of choices, click the More arrow, as shown in the picture.
Point for a preview When you rest the mouse pointer on a gallery thumbnail (hover), you see how the shape or animation effect would look when applied. If you don't like it, there's nothing to undo. Just hover over other thumbnails and click the one that you want to apply.