正解:D
The output shows that port 80 is open and running an HTTP service, indicating that the host could potentially be vulnerable to web-based attacks. The other options are not relevant for this purpose:
the host is responsive to the ICMP request, as shown by the "Host is up" message; the host is not running a mail server, as there is no SMTP or POP3 service detected; the host is not allowing unsecured FTP connections, as there is no FTP service detected.