Which command is used to enable OSPF on a Cisco router?

To enable OSPF (Open Shortest Path First) on a Cisco router, the correct command is to enter router configuration mode and use the command that initiates OSPF with a specific process ID. The structure of the command is "router ospf [process-id]," where the process ID is a unique identifier for the OSPF routing process on that router.

This command allows the router to start participating in OSPF routing, which facilitates dynamic routing based on the OSPF protocol. Once this command is executed, the router can then be configured to manage OSPF areas, interfaces, and other parameters necessary for OSPF operation.

The other options do not represent valid Cisco IOS commands for enabling OSPF. The command format and syntax are critical in the context of Cisco's command line interface, making it essential to utilize the correct command to successfully enable and configure OSPF.