Some users report that they cannot use the VPN to connect to the office via...

The correct answer is D. The firewall certificate has expired. The phrase “Your connection is not secure” is a strong browser certificate warning, not a normal VPN credential or client error. Quentin Docter’s Complete Study Guide explains that HTTPS/SSL/TLS communications rely on certificates and that each certificate has an expiration date. When certificates expire, are replaced, or are revoked, browsers may no longer trust the connection. The All-in-One Exam Guide also explains that secure web connections use digital certificates issued by trusted certificate authorities to prove the site or device identity and support encrypted communication. If the organization’s firewall, VPN portal, or SSL inspection device presents an expired certificate, browsers will warn users that the connection is not secure. Incorrect credentials would cause an authentication failure, and not using the VPN client would usually prevent connection rather than create a certificate trust warning.