Which of these scenarios allows for multiple VLAN traffic on an interface?

The scenario that allows for multiple VLAN traffic on an interface is achieved by sending tagged traffic for multiple VLANs from a single interface. This method utilizes VLAN tagging, which enables different types of traffic to coexist on the same physical interface. When VLAN tags are added to the Ethernet frames, they allow the receiving devices to identify which VLAN each frame belongs to, facilitating the transport of multiple VLANs over a single interface.

In contrast, having the interface set to untagged for both VLANs does not support multiple VLAN traffic since untagged interfaces are typically associated with a single VLAN. Using multiple interfaces to manage different VLANs allocates separate physical or logical connections for each VLAN, which could be inefficient compared to a single interface handling tagged traffic. Lastly, the assertion that every interface must be set as untagged for VLAN isolation is not accurate, as tagged interfaces are specifically designed to provide isolation and management of multiple VLANs on a single interface, thereby contradicting the premise of isolation through the untagged configuration.