Why SVI Is Needed for Inter-VLAN Routing with One Routed Interface

Why the question points to SVIs. The constraint is not simply "route traffic between subnets"; it is "multiple VLANs" with "only a single routed interface." On a multilayer switch, an SVI is a virtual Layer 3 interface tied to a VLAN, and one SVI per VLAN gives each VLAN its own default-gateway IP address. Those SVIs let the switch route between VLANs internally, so you do not need a separate physical router interface for every VLAN. The comment describing an SVI as "a virtual Layer 3 interface on a switch associated with a specific VLAN" that "allows inter-VLAN routing directly on a switch" captures the exact role. That makes D the correct answer.

Why static routing is tempting but insufficient. Option A sounds right because multiple VLANs imply separate IP subnets and therefore routing. However, a static route only adds a destination network to a routing table; it does not create a Layer 3 gateway on each VLAN or turn the single routed interface into multiple VLAN gateways. If the device already has connected interfaces or SVIs for those VLANs, static routes are not needed for the directly connected subnets. If it does not, static routes alone do not fix the missing per-VLAN interface.

Why the others lose. B (NAT) translates addresses and is unrelated to creating VLAN gateways. C (Full duplex) is a link setting that governs simultaneous transmit and receive; it has no bearing on inter-VLAN forwarding.

The practical configuration is to enable IP routing where appropriate and create an SVI for each VLAN, assigning each SVI an address in that VLAN's subnet so hosts can use it as their gateway. The single available routed interface carries the uplink or routed path, while the SVIs provide the multiple Layer 3 boundaries. Answer D is the only option that supplies those per-VLAN Layer 3 interfaces.