Question: What triggers the increment of the 'Successful serving HS-DS cell changes' measurement in a UTRAN network?
Option 1: Receipt of a RRC message `CELL UPDATE CONFIRM` from the UE to the target RNC.
Option 2: Detection of a radio link failure in the source cell, prompting a cell change.
Option 3: Receipt by the RNC of specific RRC messages (e.g., `PHYSICAL CHANNEL RECONFIGURATION COMPLETE`) from the UE, indicating a successful cell change to a different target cell.
Option 4: Completion of a handover procedure initiated by a neighboring cell's beacon signal strength increase.
Explanation: The measurement for 'Successful serving HS-DS cell changes' is incremented when the RNC receives specific RRC messages from the UE, confirming a successful serving HS-DS cell change to a different target cell. This relates to managing radio resources and handovers within UTRAN, highlighting efficient cell changes for seamless packet-switched traffic in UMTS networks.