Question: Under what condition will a transmitting AM/UM RLC entity discard an indicated RLC SDU when instructed by the upper layer (PDCP)?
Option 1: If any segment of the RLC SDU has already been transmitted over the radio link.
Option 2: If no segment of the RLC SDU has been mapped to a RLC data PDU yet.
Option 3: Immediately, regardless of the RLC SDU's transmission status.
Option 4: Only if the RLC SDU is part of a retransmission queue.
Explanation: This correct answer reflects the specified condition for discarding an RLC SDU as per the instruction from the upper layer (PDCP). The RLC entity will discard the indicated SDU only if it hasn't been processed to the point of being mapped into a RLC data PDU, ensuring that partially transmitted or already transmitted segments are handled according to other protocol rules, maintaining data integrity and transmission efficiency in E-UTRA networks.