Question: What triggers the new eNB to initiate the Retrieve UE Context procedure towards the old eNB during a UE's attempt to resume or re-establish its RRC connection?
Option 1: Receipt of the UE's initial access message with a random temporary identifier.
Option 2: Successful identification of the old eNB based on the Resume ID or Physical Cell ID received from the UE.
Option 3: Completion of the S1-AP Path Switch procedure to establish a new S1 UE associated signalling connection.
Option 4: Detection of Radio Link Failure (RLF) in the new eNB.
Explanation: The correct answer is Option 2 because the new eNB triggers this process towards the old eNB only if it can successfully identify the old eNB, based on either the Resume ID or the Physical Cell ID provided by the UE, to retrieve the UE's context for a seamless resumption or re-establishment of the RRC connection.