Question: What is the purpose of the NAS security mode control procedure in a 5G system?
Option 1: To terminate NAS signalling security between the UE and the AMF.
Option 2: To take a 5G NAS security context into use, initialise and start NAS signalling security between the UE and the AMF with corresponding 5G NAS keys and algorithms.
Option 3: To change the value of downlink NAS COUNT used in the latest SECURITY MODE COMPLETE message.
Option 4: To delete the valid KAUSF and KSEAF.
Explanation: The correct answer is Option 2 because the purpose of the NAS security mode control procedure is to establish a secure connection between the UE and the AMF by taking a 5G NAS security context into use, initialising and starting NAS signalling security with corresponding 5G NAS keys and algorithms. This ensures secure communication between the UE and the network.