Refer to the exhibit. If RC is a stub router, which entry must be injected so that it will send traffic outside the OSPF domain?

A virtual link is used to connect a disconnected OSPF area to the backbone area (Area 0), not for providing a default route to a stub router.

A sham link is used in MPLS VPNs to connect two PE routers belonging to the same VPN that have a common site via a backdoor link, to prevent routing loops and ensure optimal forwarding, not to provide a default route to a stub router.

While more specific routes are always preferred, a stub area cannot receive external specific routes. Therefore, a default route is the only viable mechanism to exit the stub area for unknown destinations.

A stub area is an OSPF area that does not accept external routes (Type 5 LSAs) and sometimes even Type 4 LSAs. To allow routers within a stub area, like RC, to reach destinations outside the OSPF domain or other OSPF areas, the ABR (Area Border Router) connecting the stub area must inject a default route (0.0.0.0/0) into that stub area. This default route directs all traffic not matching more specific routes towards the ABR.