The correct OSPF network command to be applied to R2 for establishing an OSPF neighborship is "network 20.1.1.2 255.255.0.0 area 0". This command specifies the interface's IP address and the associated wildcard mask that inversely matches the subnet mask. The area 0 denotes that this interface belongs to OSPF Area 0. The wildcard mask 255.255.0.0 allows for all interfaces within the 20.1.x.x network to participate in OSPF. References: Implementing and Operating Cisco Service Provider Network Core Technologies source documents or study guide.