How many fully established neighbour relationships exist on an Ethernet with five routers running OSPF...

When OSPF operates over a broadcast network (such as Ethernet), it uses the DR (Designated Router) and BDR (Backup Designated Router) mechanism to reduce adjacency overhead. Instead of forming full mesh adjacencies, OSPF routers establish full neighbor relationships only with the DR and BDR.

For five routers:

Each router establishes a full adjacency with the DR and the BDR.

The DR and BDR also establish adjacency with each other.

Total number of fully established adjacencies = 5 (each router to DR/BDR).

Partial adjacencies (2-way) may exist among DROther routers, but those are not fully established neighbor relationships (i.e., not exchanging LSDBs).

This design behavior is emphasized in CCDE v3.1 to avoid excessive LSDB flooding and optimize control-plane scaling.

Why other options are incorrect:

B, C, D, E: These assume full mesh adjacencies or incorrect OSPF behavior for broadcast networks.

—