Setting proper file permissions is the best method to ensure that only authorized users have access to certain files. Permissions can be adjusted to grant specific rights, such as read, write, and execute, to specific users or groups of users, thereby effectively controlling who can access the files. Biometric authentication is more related to accessing the device itself rather than individual files. Firewall rules protect against external threats and manage network traffic but do not control access to files on a computer. Full disk encryption is essential for protecting the data if the device is stolen or lost, but it does not specifically restrict file access while the operating system is running.