Question: What is the purpose of computing a new GPRS GSM Kc128 by the Mobile Equipment (ME) with a USIM in use?
Option 1: To store it in the network for future use.
Option 2: To apply it when in A/Gb mode a GEA ciphering algorithm that requires a 64-bit ciphering key is taken into use.
Option 3: To apply it when in A/Gb mode a GEA ciphering algorithm that requires a 128-bit ciphering key is taken into use.
Option 4: To delete the existing GPRS GSM Kc128 from the ME.
Explanation: The correct answer is Option 3 because, according to the text, the ME with a USIM in use shall apply the GPRS GSM Kc128 when in A/Gb mode a GEA ciphering algorithm that requires a 128-bit ciphering key is taken into use. This indicates that the purpose of computing a new GPRS GSM Kc128 is to use it for ciphering when a specific type of algorithm is employed.