Question: What is the primary purpose of the Nnef_APISupportCapability_Subscribe service operation in the 5G System (5GS)?
Option 1: To request a one-time report on the availability of a service API for a UE or group of UEs.
Option 2: To subscribe to receive continuous notifications about changes in the network topology.
Option 3: To retrieve information about the supported APIs for a specific UE or External Group ID.
Option 4: To unsubscribe from receiving notifications about service API support capabilities.
Explanation: The Nnef_APISupportCapability_Subscribe service operation allows an Application Function (AF) to subscribe to receive notification about the availability or expected level of support of a service API for a UE or group of UEs. This is evident from the description, which states that the AF subscribes to receive notifications and can choose between one-time reports or continuous reports.