Final answer:
The stage during which routers establish master-slave relationships after DR and BDR are elected for exchanging link-state information is called the Exstart state in the OSPF protocol.
Step-by-step explanation:
After DR (Designated Router) and BDR (Backup Designated Router) are elected, neighbor routers form master-slave relationships in order to establish the method for exchanging link-state information. Among the given options (a) Down, (b) Init, (c) 2-Way, (d) Exstart, and (e) Exchange, the correct answer is (d) Exstart. This stage is essential in the OSPF (Open Shortest Path First) protocol as it precedes the state where link-state advertisement (LSA) exchange occurs. During the Exstart state, the routers establish a master-slave relationship and agree on the sequence numbers for the LSA headers, ensuring a synchronized exchange afterwards.