The shortcut key used to undo the last action in a document is:

🧠 Explanation:

The shortcut key used to undo the last action in a document is **Ctrl + Z**. This command quickly reverses the most recent change, helping users correct mistakes or revert edits easily. Using this shortcut enhances productivity by allowing fast error correction without navigating through menus. It is one of the most commonly used shortcuts in various software applications.