What must be changed on Router 1 to establish an OSPF adjacency with Router 2?

For Router 1 to establish an OSPF adjacency with Router 2, it is crucial that both routers have interfaces in the same subnet. This means that their IP addresses must belong to the same network segment, which requires matching subnet masks. If the subnet mask on Router 1's FastEthernet 0/0 interface is not configured correctly to enable both routers to see each other as being on the same network, they will not be able to form an adjacency.

Changing the subnet mask on Router 1's FastEthernet 0/0 interface to 255.255.255.0 is appropriate if Router 2's interface on the same segment is also configured with a corresponding IP address that fits within that subnet. By ensuring both routers use compatible subnet masks, they can recognize that they are part of the same network and thus initiate the OSPF adjacency process correctly.

In contrast, adjusting the router ID is typically a need after the adjacency has been established but does not directly affect the formation of the adjacency in the first place. Similarly, changing the OSPF area or interface type might be relevant if you're dealing with issues related to OSPF's area design or network topology, but those actions would not solve the fundamental problem