The Linux user that is used by vsftpd to perform file system operations for anonymous FTP users is the one specified in the configuration option ftp_username. This option defines the local user that vsftpd will run as when handling anonymous FTP requests. By default, this option is set to ‘ftp’, which is a predefined user account that has limited privileges and access to the FTP server. The other options are incorrect for the following reasons:

A. The Linux user which runs the vsftpd process. This is false because vsftpd does not use the same user that runs the daemon to handle anonymous FTP requests. Instead, it switches to the user specified by ftp_username, which is usually a different and less privileged user than the one that starts the vsftpd process.

B. The Linux user that owns the root FTP directory served by vsftpd. This is false because vsftpd does not use the owner of the root FTP directory to perform file system operations for anonymous FTP users. The owner of the root FTP directory may or may not be the same as the user specified by ftp_username, but it does not affect the behavior of vsftpd for anonymous FTP requests.

C. The Linux user with the same user name that was used to anonymously log into the FTP server. This is false because vsftpd does not use the user name that was used to anonymously log into the FTP server to perform file system operations. The user name that is used for anonymous FTP login is usually ‘anonymous’ or ‘ftp’, but it does not correspond to any actual Linux user account on the system. Instead, vsftpd maps these user names to the user specified by ftp_username, which is the one that actually performs the file system operations.

D. The Linux user root, but vsftpd grants access to anonymous users only to globally read-/writeable files. This is false because vsftpd does not use the root user to perform file system operations for anonymous FTP users. The root user is the most powerful and privileged user on the system, and using it for anonymous FTP requests would pose a serious security risk. Instead, vsftpd uses the user specified by ftp_username, which is usually a low-privileged user that has limited access to the FTP server. The option anon_world_readable_only controls whether vsftpd only allows anonymous users to access files that have global read permissions, regardless of the permissions of the user specified by ftp_username.

References: LPIC-2 202 exam objectives, LPIC-2 202-450 Exam Prep: Network Configuration, Reference Manual For OpenVPN 2.4, linux - What is the anonymous user in vsftp? - Super User

How To Set Up vsftpd for Anonymous Downloads on Ubuntu 16.04