Question: During a RRC connection re-establishment to a cell in the previous monitored set, what triggers the UE to send a RRC CELL UPDATE message with a specific cause value after moving to CELL_FACH state?
Option 1: Expiration of TRE-ESTABLISH timer due to radio link failure
Option 2: Receipt of a new system information block from the target cell
Option 3: Completion of UE's handover measurement report
Option 4: Detection of a better serving cell in the active set
Explanation: When a radio link failure occurs, causing the UE to transition from CELL_DCH to CELL_FACH state, the UE sends a RRC CELL UPDATE message with the cause value 'radio link failure' within a specified time frame. This is crucial for the network to promptly respond to the radio link failure and re-establish the connection efficiently.