Which method must be used to backup existing files before a clean installation?

To ensure all existing files are preserved before performing a clean installation of an operating system, it is essential to follow the backup process. This involves creating copies of your important files—such as documents, photos, and application data—onto an external storage device or a cloud-based service. By backing up your files, you safeguard against data loss that can occur during the clean installation, which typically erases the existing operating system and all files on the primary storage drive.

Utilizing the backup method not only protects your critical data but also provides a means to restore it after the installation is complete. This practice is crucial as a clean installation often requires formatting the drive, resulting in permanent loss of all data stored on it.

The other methods mentioned do not fulfill the purpose of backing up existing files effectively. For instance, a direct installation does not take into account data preservation, and system restore is primarily used for reverting the system's state without affecting files. Similarly, running disk cleaning utilities is more concerned with optimizing performance and clearing unnecessary files rather than backing up important data.