What command is used to enter EIGRP configuration mode on a router?

The command used to enter EIGRP configuration mode on a router is "router eigrp as-number." This command allows you to define a new EIGRP routing process and specify the autonomous system number, which is crucial for the router to participate in EIGRP routing.

When you issue this command in global configuration mode, you're effectively telling the router to start using EIGRP for routing decisions based on the specified autonomous system. Adding an autonomous system number is essential because it identifies the EIGRP routing domain to which the router belongs and allows for proper operation and communication with other EIGRP routers.

The other choices do not represent the correct syntax for entering EIGRP configuration mode. They either lack necessary keywords or use incorrect formats that do not conform to Cisco's command-line structure for routing protocol configuration.