A virtual link between two ABRs remains down after configuration. One ABR has the transit area configured as an NSSA. Which statement best explains the failure?

Virtual links are designed to connect an OSPF area to the backbone (Area 0) when direct physical connectivity isn't possible. They achieve this by traversing a non-backbone area. This transit area must be a normal OSPF area, meaning it cannot be a stub, totally stubby, or NSSA (Not-So-Stubby Area). NSSAs, like stub areas, restrict the flooding of external LSAs (Type 5 and Type 7) and therefore cannot carry the transit traffic required for a virtual link to function correctly. Different router IDs are permissible, as are various authentication methods (not just MD5), and while MTU mismatches can cause issues, they typically manifest as adjacency problems rather than outright virtual link failure due to area type.