In which of the following scenarios can a neighbor relationship not be established between two...

In IS-IS:

Level-1 routers can only form neighbor relationships with other Level-1 routers in the same area .

Level-2 routers can form neighbor relationships with other Level-2 routers regardless of area ID .

Level-1-2 routers can form relationships at both levels depending on the peer ' s configuration.

So, if two Level-1 routers have different area addresses , they cannot establish a neighbor relationship.

Reference from Huawei HCIP-Datacom-Core Technology Study Guide:

“Level-1 routers must have the same area address to establish neighbor relationships. Level-2 routers can peer across different areas.”

(Chapter: IS-IS Protocol Principles – Section: Neighbor Establishment)