Question: What marks the end of the cell re-selection delay for a User Equipment (UE) transitioning from one Radio Access Technology (RAT) to another, specifically when re-selecting to a Time Division Duplexing (TDD) cell?
Option 1: Completion of the RRC CELL UPDATE message transmission over the UTRAN
Option 2: Initiation of sending the SYNCH-UL sequence for the RRC CELL UPDATE message to the UTRAN
Option 3: Receipt of the first response from the target TDD cell
Option 4: Completion of the handover procedure to the new TDD cell
Explanation: According to the defined metrics for cell re-selection delay, when transitioning to a TDD cell, the delay ends at the moment the UE begins sending the SYNCH-UL sequence. This is specifically for initiating the RRC CELL UPDATE message transmission to the UTRAN, marking the transition's key milestone in terms of radio resource management timing.