Which command sequence sets up a GRE tunnel interface on a Cisco IOS router?

The command sequence for setting up a GRE tunnel interface on a Cisco IOS router involves entering global configuration mode and then specifying the interface type as a tunnel, followed by assigning an IP address to that tunnel interface. In this case, the correct answer specifies the use of "interface tunnel0," which is a valid option for establishing a GRE tunnel, and it assigns an appropriate IP address of "172.16.99.2" with a corresponding subnet mask of "255.255.255.0."

Using "tunnel0" is common because GRE tunnels are often referred to with the first logical tunnel interface starting at 0. The IP address chosen should be part of the same subnet that will be used on the other end of the GRE tunnel to ensure proper communication and routing through the tunnel.

Other options may contain correct elements but fail to meet the criteria for properly defining the tunnel or using the appropriate addresses. For instance, specifying "tunnel1" could be valid, but the combination of address and subnet mask in other options may either not conform to the expected IP addressing practices associated with tunneling or could be referencing an IP address range not suitable for this implementation.