Answer:
D. BGP
Explanation:
In order to exchange routing information with another AS (Autonomous Systems) only, exterior routers use an exterior routing protocol, like BGP (Border Gateway Protocol) or EGP (Exterior Routing Protocol).
BGP is an application-layer protocol in the TCP/IP suite, that uses a well-known TCP port (179) in order to deliver information reliably.
It is important to note that in order to exchange routing data within his own AS, the router uses an interior routing protocol, like OSPF.
