Which of the following commands is used to copy files?

The command that is used to copy files in Linux is 'cp'. This command allows users to create duplicate files or directories, making it an essential tool for file management in the Linux operating system. When using 'cp', you can specify various options to control aspects of the copying process, such as whether to copy directories recursively or preserve file attributes.

In contrast, 'move' is used to move files from one location to another and does not create a copy of the original file. The 'copy' command, while it might sound intuitive, is not a recognized command in Linux for file copying; it is often associated with other operating systems like Windows. 'Extract' typically refers to commands used for unpacking compressed files or archives, rather than copying them. Thus, 'cp' is the correct command for the task of copying files in the Linux environment.