Which description of a vFC interface is true?

A vFC interface can only be a member of a single VSAN; VSAN membership is one-to-one per vFC interface, not multi-VSAN.

A vFC interface is a virtual Fibre Channel interface that has no physical transport of its own. It must be explicitly bound to an FCoE-enabled physical Ethernet or EtherChannel interface on the Nexus switch, which provides the actual Layer 2 transport for encapsulated FC frames. This binding is a required configuration step and is what allows the vFC to participate in the FCoE fabric.

Private VLANs are a Layer 2 Ethernet isolation feature and are not applicable to vFC interfaces, which operate within the FC/VSAN domain rather than the VLAN domain.

vFC interfaces are associated with VSANs, not FCoE VLANs; the FCoE VLAN is configured on the bound Ethernet interface and mapped to a VSAN, but the vFC itself joins a VSAN, not a VLAN.