Question: What triggers the addition of each path update to the relevant measurement in the context of Path Update Request at Secondary Node Additions?
Option 1: Transmission by the MME of an E-RAB modification indication message to MN.
Option 2: Receipt by the SgNB of a Handover Request message from the MeNB.
Option 3: Detection by the UE of a change in the serving cell's identity.
Option 4: Transmission by the MN of an E-RAB modification indication message to MME at Secondary Node Additions.
Explanation: According to the provided text, each path update is added to the relevant measurement on transmission by the MN of an E-RAB modification indication message to MME at Secondary Node Additions. This indicates that the correct trigger for adding each path update to the measurement is indeed the transmission of this specific message.