Question: What triggers the counting of all E-RABs for a UE in the measurement of the number of E-RABs attempted to establish for incoming HOs?
Option 1: Receipt by the eNB of a X2AP HANDOVER REQUEST or S1AP HANDOVER REQUEST message, or transmission by the eNB of the RRCConnectionReconfiguration message to the UE triggering the intra-eNB handover.
Option 2: Transmission by the eNB of the RRCConnectionReconfiguration message to the UE triggering the inter-eNB handover.
Option 3: Receipt by the eNB of a X2AP HANDOVER REQUEST or S1AP HANDOVER REQUEST message, but not transmission by the eNB of the RRCConnectionReconfiguration message to the UE triggering the intra-eNB handover.
Option 4: Transmission by the eNB of the RRCConnectionReconfiguration message to the UE triggering the inter-RAT handover.
Explanation: According to the text, counting of all E-RABs for a UE is triggered when the eNB receives a X2AP HANDOVER REQUEST or S1AP HANDOVER REQUEST message, or when it transmits the RRCConnectionReconfiguration message to the UE triggering the intra-eNB handover. This ensures that all E-RABs are counted for this measurement to the target E-UTRAN cell.