Exam NSE4_FGT_AD-7.6 Topic 2 Question 55 Discussion

According to the FortiOS 7.6 Administrator Guide and the specific behavior of the SD-WAN GUI, here is the technical breakdown:
SD-WAN Zone Hierarchy and UI Elements: In the FortiGate GUI, SD-WAN zones that contain member interfaces are displayed with a plus (+) icon next to the checkbox. This icon allows administrators to expand the zone and view the specific physical or logical interfaces assigned to it.
Analysis of the "Underlay" Zone: In the provided exhibit, the virtual-wan-link and overlay zones both feature the plus (+) expansion icon, indicating they have active members. The Underlay zone, however, lacks this icon and displays a red status icon. This is the visual indicator in FortiOS that the zone is currently empty and contains no member interfaces.
Mandatory Zone Membership: In FortiOS 7.x, every SD-WAN member interface must be assigned to a zone.
It is not possible for an interface to be an "SD-WAN member" (as shown in the legend with port2 and port3) without being assigned to a zone. Since port2 and port3 are listed in the legend, they are indeed assigned to one of the other expanded zones (likely virtual-wan-link or overlay), making Option D incorrect.
Default Zone Behavior: While FortiOS 7.6 often creates default zones like virtual-wan-link, underlay, and overlay during certain configuration wizards or by default in newer versions, they are distinct entities. There is no single "default" zone that acts as a global catch-all in the way Option C suggests.
Immutability of System Zones: While certain system-defined zones have restrictions, the primary focus of this specific exhibit is the current membership state, which clearly shows the Underlay zone is empty.