Question: What triggers the Mobile Equipment (ME) to add a Closed Subscriber Group (CSG) ID to the Allowed CSG list?
Option 1: Receipt of a TRACKING AREA UPDATE ACCEPT message from the E-UTRAN for any cell.
Option 2: Successful acceptance by the E-USS of a TRACKING AREA UPDATE REQUEST message sent by the ME for a cell with a specific CSG ID.
Option 3: Automatic PLMN selection mode initialization on the UE.
Option 4: Detection of a cell with csg-Indication set to TRUE during idle mode.
Explanation: This is correct because the ME adds the CSG ID to the Allowed CSG list when the CSG ID belongs to the cell where the ME sent an accepted TRACKING AREA UPDATE REQUEST message, authorizing operation within that CSG.