Which roles are valid for OSPF intra-area or inter-area routers?

In the context of OSPF (Open Shortest Path First) routing, understanding the roles of various types of routers is essential. An internal router is defined as a router that is located entirely within a single OSPF area. This designation allows internal routers to perform various routing functions specific to their designated area, sharing link-state information only with routers within that same area. This role is foundational to how OSPF organizes its routing tables and disseminates information, making option A correct.

In contrast, an Area Border Router (ABR) specifically connects two or more areas, one of which must be the backbone area (Area 0); therefore, stating that it does not connect to the backbone is incorrect. A backbone router is not required to have four interfaces; its primary role is to form the backbone of an OSPF network rather than being defined by the number of interfaces. Lastly, an Autonomous System Boundary Router (ASBR) can exist as either an internal router or an external router depending on its configuration with respect to interconnecting OSPF with non-OSPF networks, which makes the claim that it can only be an internal router incorrect. Thus, internal routers are indeed valid roles for OSPF, affirming why option