Explanation
The FTP names that are recognized as anonymous users in vsftpd when the option anonymous_enable is set to yes in the configuration file are anonymous and ftp. These are the default names that vsftpd accepts as valid anonymous users, and they can be used interchangeably. The anonymous user does not need to enter a password, but can optionally enter an email address as a password. The anonymous user has limited permissions and can only access the directory specified by the anon_root directive, which is usually /var/ftp or
/srv/ftp. The anonymous user can download files from the server, but cannot upload or modify files, unless the anon_upload_enable and anon_mkdir_write_enable directives are also set to yes.
The other options are not recognized as anonymous users in vsftpd. Option C is incorrect, because vsftpd will reject any username that does not belong to an existing user or an anonymous user. Option D is incorrect, because nobody is a system user that has no permissions and is not related to FTP. Option E is incorrect, because guest is not a valid FTP name, unless the guest_enable directive is set to yes, which will make all non-anonymous users log in as the guest_username, which is usually ftp.
References:
vsftpd.conf - config file for vsftpd
How To Set Up vsftpd for Anonymous Downloads on Ubuntu 16.04
How to Configure vsftpd FTP Server on CentOS 8