A and B are correct. The syntax to configure a default route is ip route 0.0.0.0 0.0.0.0 [next_hop_address|egress_int]. C and D are incorrect. They are using LAN addresses for the next hop, not the serial0/0 interface. Solution A and B are correct. The syntax to configure a default route is ip route 0.0.0.0 0.0.0.0 [next_hop_address|egress_int]. C and D are incorrect. They are using LAN addresses for the next hop, not the serial0/0 interface..