Question: What triggers the Master Node (MN) to perform a Security Context (SC) modification procedure to deliver a fresh Key Separation Number (KSN) to the Secondary Node (SN)?
Option 1: Periodic timer expiration
Option 2: UE-initiated security context update request
Option 3: SN-initiated KSN synchronization request
Option 4: MN's decision to update the KSN for any reason
Explanation: The Master Node (MN) may update the Key Separation Number (KSN) for any reason, which triggers the MN to perform a Security Context modification procedure to deliver the fresh KSN to the Secondary Node (SN). This ensures that both nodes have the same KSN value, maintaining security and integrity in the 5G system.