The system administrator wishes to use the pam_listfile.so module to restrict which users are allowed to login via SSH. Which line will configure this behaviour?

`sense=deny` would prevent listed users from logging in, which is the opposite of the desired 'restrict which users are allowed' (implying a whitelist); `onerr=succeed` is also a security risk, allowing access if the file is missing or unreadable.

While `sense=allow` is correct, `onerr=succeed` is a security risk as it would allow all users to log in if the allow file is missing or unreadable, compromising the restriction.

This line correctly uses `pam_listfile.so` to check the `user` (`item=user`), `allow` (`sense=allow`) users listed in `/etc/sshd/sshd.allow` (`file=/etc/sshd/sshd.allow`), and critically, `fail` (`onerr=fail`) if the file cannot be accessed, which prevents an insecure fallback and ensures security.

`sense=deny` would block listed users, not allow only a specified list; while `onerr=fail` is generally more secure, the `sense` parameter is incorrect for an allowlist.