正解:D
OSPF (Open Shortest Path First) adjacency between R1 and R2 fails due to a timers mismatch. In OSPF, it's crucial for routers to have matching Hello and Dead intervals to form an adjacency. Ifthese timers do not match, OSPF routers cannot establish an adjacency, leading to failure in exchanging routing information. References: Implementing and Operating Cisco Service Provider Network Core Technologies source documents or study guide