If no loopback or explicit router ID is configured before starting OSPF, how is the router ID chosen?

When OSPF starts without a configured router ID or a loopback interface, it automatically selects a router ID. The selection process prioritizes loopback interfaces first. If no loopback interfaces exist, OSPF then examines all active (up/up) physical interfaces. From these, it chooses the highest IP address as the router ID. This ensures a unique identifier for the router within the OSPF domain, which is crucial for proper OSPF operation and neighbor relationships. The other options are incorrect because OSPF does not increment IP addresses, use 0.0.0.0 as a router ID, or fail to run simply due to the absence of a manually configured router ID.