Which configuration command is required to enable EIGRP on a router?

To enable Enhanced Interior Gateway Routing Protocol (EIGRP) on a router, the configuration command that must be used is "router eigrp [AS number]." This command enters EIGRP configuration mode, allowing the administrator to specify the Autonomous System (AS) number that the router will use for EIGRP routing. The AS number is essential because it uniquely identifies the EIGRP routing domain to which the router belongs.

In this command, the AS number can be any value from 1 to 65535, and it must match across all routers within the same EIGRP routing domain for them to exchange routing information effectively. Upon entering this command, you can then proceed to configure EIGRP parameters such as network statements, router configurations, and various EIGRP features.

The other options either do not represent valid configuration commands or do not fulfill the requirement to enable EIGRP properly. Therefore, using "router eigrp [AS number]" is the necessary step to enable EIGRP and start routing within that defined Autonomous System.