Which protocol advertises WAN Edge routes on the service side?

BGP (Border Gateway Protocol) is the correct answer because in Cisco SD-WAN, BGP is the primary routing protocol used by vEdge or cEdge (WAN Edge) devices to advertise service-side routes (routes learned from the local network) into the overlay network. These routes are then distributed to other WAN Edge devices via OMP (Overlay Management Protocol). While OSPF, EIGRP, and ISIS are all interior gateway protocols (IGPs) that can be used on the service side to learn routes from the local network, they do not directly advertise these routes into the SD-WAN overlay. Instead, the WAN Edge device redistributes routes learned from these IGPs into BGP, which then advertises them into the overlay.