What command checks for overwritten protected system files and replaces them with the original versions?

The command that checks for overwritten protected system files and replaces them with the original versions is the System File Checker, commonly referred to as SFC. This utility is a crucial tool built into Windows operating systems that scans for corruptions in system files and, when necessary, restores them from a local cache or installation media.

When executed, SFC examines the integrity of all protected operating system files and repairs any discrepancies it finds, ensuring that the system is running the original, unaltered files as intended by the operating system's installation. This is particularly useful in troubleshooting issues stemming from corrupted or missing system files that could affect system stability or performance.

The other options serve different purposes. For instance, chkdsk is designed to check the file system and the file system metadata of a volume for logical and physical errors but does not specifically target system files for repair. Msconfig is a system configuration utility that helps in managing startup programs and services but does nothing for the integrity of system files. Lastly, xcopy is a command-line utility used for copying files and directories, including their subdirectories, but does not check or replace system files.

Thus, SFC is the correct choice for ensuring that your operating system retains the integrity of its protected files.