Final answer:
The shortcut key to select everything in a document is Ctrl + A, which is widely used across different applications.
Step-by-step explanation:
The shortcut key that enables you to select everything in a document is Ctrl + A. This command is commonly used in various applications, such as word processors, spreadsheets, and web browsers, to quickly select all the content or elements within the current view or document. The other options, such as Ctrl + Home, Ctrl + End, and Ctrl + up or down arrow key, provide different navigational functions but do not select all content.