how do the internal routers within the Transit AS forward packets destined for the external networks using a scalable solution?

In a Transit AS, how do the internal routers within the Transit AS forward packets destined for the external networks using a scalable solution?

A.
Using the default route.

B.
Using the IGP routes where the external networks are redistributed into the IGP by the edge routers.

C.
Using the EBGP routes where the external networks are redistributed into the IBGP by the edge routers.

D.
Using the IBGP routes, then using recursive lookup based on IGP information to resolve the BGP next-hop.