Final answer:
To resolve routers stuck in the "ExStart" state in OSPF, check the hello timers, authentication settings, and also ensure that the MTU sizes match, as these can all prevent routers from establishing adjacency.
Step-by-step explanation:
If routers are stuck in the "ExStart" state in OSPF, you should primarily check three things: hello timers, authentication settings, and MTU size. Although not explicitly listed among the choices, mismatched MTU sizes can also cause this issue. The ExStart state is part of the OSPF process where routers establish adjacency. During this process, routers exchange Hello packets to agree on certain parameters. If the hello timers are mismatched, routers may not recognize each other as neighbors. Similarly, improper authentication configurations will prevent the establishment of a neighbor relationship because OSPF routers must have the same authentication type and credentials to form an adjacency. Lastly, if routers have different MTU settings, they cannot move past the ExStart state.