VLAN Mode is a more efficient way to connect a FortiExtender to a FortiGate than CAPWAP Mode. This is because VLAN Mode does not require the FortiExtender to send additional control traffic to the FortiGate.
The other options are not correct.
* A. Add a switch between the FortiGate and FEX. This will add overhead to the network, as the switch will need to process the traffic.
* B. Enable CAPWAP connectivity between the FortiGate and the FortiExtender. This will increase the overhead on the FortiGate, as it will need to process additional control traffic.
* D. Add a VLAN under the FEX-WAN interface on the FortiGate. This will not affect the overhead on the FortiGate.
http://docs.fortinet.com/document/fortiextender/7.0.3/admin-guide-fgt-managed/394272/vlan-mode
http://docs.fortinet.com/document/fortiextender/7.0.3/admin-guide-fgt-managed/618684/vlan-mode-and- performance