Question: In a TDD system, what triggers a UE to block a currently used Transport Format Combination in the uplink for packet data services like Interactive or Background with 64kbps?
Option 1: Receipt of a new TFC selection message from the eNodeB.
Option 2: Insufficient UE output power to support the current TFC.
Option 3: Change in the Quality of Service (QoS) requirements for the ongoing session.
Option 4: Expiration of a predefined timer set for TFC re-evaluation.
Explanation: This is correct because, as per general requirements on TFC selection for packet data services, the UE must adjust its transmission to match its current power capabilities. If the UE's output power is insufficient to support the currently selected TFC, it will block that TFC to prevent transmission errors or failures, ensuring reliable communication within its power limits.