Border Gateway Protocol (BGP): BGP is the most commonly used routing protocol for interconnecting WANs, especially across the internet. It is used for exchanging routing information between autonomous systems (AS), making it the backbone protocol for large-scale WANs.

IGP (A): Interior Gateway Protocols like OSPF and EIGRP are typically used within a single AS, not between them.

EIGRP (B): While it is efficient, EIGRP is primarily used for intra-domain routing and not ideal for WAN interconnection.

OSPF (D): While OSPF can be used for WANs, it is not as common as BGP for inter-AS communication.