The correct answer is D. For FortiGate VPN integration, FortiNAC-F depends on syslog from FortiGate to receive VPN user, IP address, and session information. The FortiNAC-F study guide states that after a remote user successfully authenticates and establishes a VPN connection, FortiGate sends user, IP, and session information to FortiNAC-F using syslog. This keeps FortiNAC-F aware of the VPN session so it can apply the correct access control state and update FortiGate when the device becomes trusted.

Option A is wrong because SNMP traps are commonly used for infrastructure events, link traps, or third-party event inputs, but this VPN workflow uses FortiGate syslog. Option B is wrong because RADIUS accounting can update session information in some NAC workflows, but the FortiGate VPN integration described in the guide uses syslog. Option C is wrong because Security Fabric integration is not the required mechanism for keeping FortiNAC-F updated with VPN session details in this scenario.