Which of the following is NOT a requirement for forming an OSPF adjacency?

To form an OSPF (Open Shortest Path First) adjacency, certain requirements must be met to ensure that routers can communicate effectively within the network. The element that is not a requirement among the options provided pertains to the diversity of OSPF area types.

For OSPF adjacency, routers must be in the same subnet to facilitate direct Layer 2 communication, which is essential for establishing neighbor relationships. Additionally, matching Hello and dead intervals are important because these intervals dictate the frequency of OSPF messages sent between routers, impacting their ability to detect an active neighbor. Each router also needs to have a unique router ID to establish clear identities among multiple OSPF routers within the same area.

However, having different OSPF area types does not prevent the formation of adjacencies. Routers can establish adjacency as long as they are in the same OSPF area. For instance, even if routers are in areas of different types (such as stub area or normal area), they can still form an adjacency if they meet the other essential criteria. Thus, the requirement to have different area types does not play a role in forming OSPF adjacencies, making this choice the correct answer for the question.