Which Tunnel1 configuration on R2 establishes the GRE tunnel to R1 (use loopbacks and avoid MTU issues)?

The correct configuration uses the loopback interfaces (10.10.2.2 on R2 and 10.10.1.1 on R1) as the tunnel source and destination, as specified in the question to "use loopbacks." It also correctly sets ip mtu 1400 and ip tcp adjust-mss 1360 to avoid MTU issues, which is a common best practice for GRE tunnels. The first incorrect option uses physical interface IP addresses (192.168.20.1 and 192.168.10.1) instead of loopbacks for the tunnel source and destination. The third incorrect option uses an ip mtu 1500, which is typically too large for a GRE tunnel and would likely cause fragmentation issues, despite the ip tcp adjust-mss 1360. The fourth incorrect option incorrectly mixes a physical interface IP (192.168.20.1) with a loopback IP (10.10.1.1) and also uses an ip mtu 1500.