Which command is used to enable RIPv2 in the router configuration?

The command to enable RIPv2 in the router configuration is "version 2." This command is used within the RIP routing configuration mode to specify that the router should operate using RIPv2, which is the second version of the Routing Information Protocol. RIPv2 includes enhancements over its predecessor, RIPv1, such as support for subnetting and the use of multicast for sending updates.

When you're in router configuration mode, entering "version 2" tells the router to enable the functionalities and improvements provided by RIPv2. This includes features like sending routing updates to the multicast address 224.0.0.9 and supporting Classless Inter-Domain Routing (CIDR) through the inclusion of subnet mask information in its updates, allowing for more efficient use of IP addresses.

The other options do not represent valid commands for enabling RIPv2 within the router configuration. The command structure is specific and must follow the syntax recognized by the Cisco IOS (Internetwork Operating System). Thus, "version 2" is the appropriate and correct command for enabling RIPv2 on a Cisco router.