The correct answer is D. You can configure forward domain IDs for each network. The study guide explains that in FortiGate transparent mode, ''all interfaces belong to the same broadcast domain, even interfaces with different VLAN IDs'' and then states that you should ''use this command to subdivide into multiple broadcast domains'' with set forward-domain <domain_ID>. It further explains that ''interfaces with the same domain ID belong to the same broadcast domain'' and ''traffic arriving on one interface is broadcast only to interfaces in the same forward domain ID.'' This is exactly the mechanism used to separate one internal network from another and improve segmentation.

The other options do not match this requirement. Universal ZTNA is described as controlling user access to applications, not segmenting two internal OT networks. An explicit software switch is for controlling intra-switch or intra-VLAN traffic inside the same software switch broadcast domain, which is more aligned with microsegmentation than separating two routed internal networks. One traffic VDOM does not create segmentation by itself; segmentation with VDOMs requires multiple VDOMs, not one. Therefore, the best choice for segmenting network 1 and network 2 in this scenario is to assign separate forward domain IDs.