Option B configures area 7 as a not-so-stubby area (NSSA) with the no-summary option on the area border router (ABR) R4. This allows the injection of external routes from EIGRP into the OSPF domain as type 7 LSAs, while preventing the propagation of inter-area summary LSAs into area 7. The no-summary option also generates a default summary route for area 7, which can be used by R9 to reach the internet2.
Option A configures area 7 as a stub area, which does not allow any external routes or type 7 LSAs in the area. This prevents R9 from learning the EIGRP routes and accessing the internet3.
Option C configures area 7 as a NSSA without the no-summary option on R4. This allows the injection of type
7 LSAs into the area, but also allows the propagation of inter-area summary LSAs into the area. However, this option does not generate a default summary route for area 7, which means R9 has no route to reach the internet4.
Option D configures area 7 as a NSSA with the default-information originate option on R4. This allows the injection of type 7 LSAs into the area, but also allows the propagation of inter-area summary LSAs into the area. The default-information originate option generates a type 7 default route for area 7, which can be used by R9 to reach the internet. However, this option is redundant and less efficient than Option B, because it injects both a type 3 and a type 7 LSA for the default route into the area5.