The "d-wan" zone in FortiGate SD-WAN configuration is the default SD-WAN zone created when SD-WAN is enabled. This zone contains all the interfaces assigned to SD-WAN and is essential for the functionality of the SD-WAN feature. The "d-wan" zone cannot be deleted because it is required for SD-WAN operations.
Option A is incorrect because the underlay zone does not contain port1. Options B and D are incorrect because they incorrectly describe the configuration of zones.
References:
* FortiOS 7.4.1 Administration Guide: SD-WAN Zone Configuration