Final answer:
The keyboard shortcut in Photoshop for undoing an action one time is Command plus Z (or Control plus Z for Windows users).
Step-by-step explanation:
The keyboard shortcut in Photoshop for undoing an action one time is Command plus Z for Mac users, and Control plus Z for Windows users. This shortcut allows you to undo a single action, and if you wish to undo multiple steps, you can use Command + Option + Z on a Mac or Control + Alt + Z on Windows to step back through your action history. It's very important for Photoshop users to know and use these undo shortcuts effectively to improve their workflow.