Final answer:
A command button in a graphical user interface is used to perform an action, such as saving a document or opening a file.
Step-by-step explanation:
The graphical user interface (GUI) element used to perform an action is the command button.
This is an interactive component that, when clicked by the user, initiates a specific action predetermined by the software program's code.
For instance, clicking a command button in a word processor may initiate actions such as saving a document, opening a new file, or printing the current page. Other GUI elements like checkboxes and drop-down menus serve different purposes, such as selecting options or displaying a list of choices, respectively.