Which button would you click to revert changes made to an applied background?

The option to click in order to revert changes made to an applied background is the "Undo" button. This functionality allows users to reverse the most recent action taken in PowerPoint, including changes to slide backgrounds. When you've applied a new background and decide you want to revert to the previous one, using the Undo command will effectively restore the slide to its state before the background change.

This action is part of PowerPoint's general control scheme, where users can sequentially undo their most recent changes, which is especially handy when experimenting with design elements. If you decide you don't like a particular adjustment, such as changing the slide's background, the Undo command enables you to easily revert that specific change without affecting other alterations you've made to the slides.

The other options do not provide the same functionality. For instance, resetting generally refers to a complete return to the default of a specific element rather than just reversing a recent change, and applying to all would set the selected background to every slide, not revert a change. The cancel option usually refers to closing out a dialog box without saving changes, rather than reverting an action already taken.