Question: What triggers a User Equipment (UE) to start sending preambles on the Physical Random Access Channel (PRACH) for a CELL UPDATE message, with a cause value of 'cell reselection', in the context of cell re-selection delay?
Option 1: A change in cell levels that prompts the UE to camp on a new cell.
Option 2: Receipt of a dedicated paging message from the new cell.
Option 3: Expiration of a timer set during the initial cell attachment process.
Option 4: Completion of a successful handover procedure initiated by the network.
Explanation: The correct answer is Option 1 because, according to the provided context, the cell re-selection delay is specifically tied to the time it takes for a change in cell levels to result in the UE camping on a new cell and subsequently initiating communication with the new cell by sending preambles on the PRACH for a CELL UPDATE message. This action is directly triggered by the UE's decision to reselect a cell based on changed cell signal levels, making Option 1 the most accurate choice.