An administrator is configuring FortiNAC-F to manage FortiGate VPN users.

The correct answer is C . The FortiNAC-F study guide explains that all VPN hosts are initially treated as unauthorized. For those unauthorized VPN hosts, the FortiGate firewall policy must allow traffic only to and from the FortiNAC-F VPN isolation interface and deny all other traffic. This forces the connecting VPN client into the FortiNAC-F validation process, including captive portal presentation and FortiNAC-F agent communication or download.

Options A and B are incorrect, and they appear duplicated in the question. The client is not supposed to be granted access only to the production DNS server while still unauthorized. FortiGate assigns DNS during VPN connection setup, with production DNS as primary and the FortiNAC-F VPN isolation interface as secondary, but the unauthorized firewall policy restricts useful access to FortiNAC-F so that validation can occur. Option D is wrong because the VPN client has already connected to the FortiGate VPN service; the authorization workflow requires access to the FortiNAC-F VPN isolation interface , not merely the FortiGate VPN interface.