Which command enables IPv6 routing on the router?

The command that enables IPv6 routing on the router is "ipv6 unicast-routing." This command is essential for allowing the router to perform IPv6 routing functions, enabling it to forward IPv6 packets between interfaces. When this command is issued in the global configuration mode, it tells the router to enable its IPv6 routing protocols, which are necessary for it to manage and direct IPv6 traffic effectively.

For a router to handle IPv6 traffic correctly, this command is required so that the router can participate in the IPv6 routing process. Without enabling unicast routing specifically, the router would not recognize or forward IPv6 packets, limiting its routing capabilities to only IPv4.

Other options such as "ipv6 enable" and "ipv6 routing" either do not exist in Cisco's command-line interface or do not specifically enable the routing functionality required for IPv6. "ipv6 config" is also not a recognized command for activating IPv6 routing. Therefore, "ipv6 unicast-routing" is the appropriate command that activates the necessary features for IPv6 packet handling in the routing context.