Question: In the context of Generic Authentication Architecture (GAA) and Generic Bootstrapping Architecture (GBA), what is the primary purpose of the `Nhss_GbaSDM_Notification` service operation initiated by the Home Subscriber Server (HSS)?
Option 1: To request GBA subscriber data from a Network Function (NF) for initial setup.
Option 2: To update a Network Function (NF) about changes to subscribed GBA subscriber data types.
Option 3: To authenticate a User Equipment (UE) attempting to access GBA services.
Option 4: To synchronize GBA keys between the HSS and multiple NFs for redundancy.
Explanation: The correct answer reflects the service operation's description as a mechanism for the HSS to notify an NF of any changes to the GBA subscriber data types that the NF has subscribed to, ensuring the NF remains updated with the latest information.