正解:C
The configuration required to set up a GRE tunnel between the HQ and BR routers is found in Option C. This configuration specifies the tunnel source as the IP address of the interface on the HQ router, and the tunnel destination as the IP address of an interface on the BR router. The ip address command assigns an IP address to the tunnel interface itself. This setup allows for the encapsulation and de-encapsulation of traffic between the two routers, enabling them to communicate over the GRE tunnel.