正解:B
When using Cisco ISE for a guest portal with the default configuration, if a warning appears during the testing phase, it typically indicates an issue with the server's certificate. In most cases, this warning occurs because the server is using a self-signed certificate which is not trusted by user devices by default. Users will see a warning message in their browsers when trying to access the guest portal because their devices do not recognize the issuer of the self-signed certificate as a trusted authority.