Question: In an E-UTRAN, which entity sends the UE Radio Capability ID Mapping Response message, and what is its primary direction?
Option 1: Sent by gNB, Direction: eNB → gNB
Option 2: Sent by eNB, Direction: eNB → gNB
Option 3: Sent by UE, Direction: UE → eNB
Option 4: Sent by MME, Direction: MME → eNB
Explanation: The correct answer is Option 2 because the UE Radio Capability ID Mapping Response message is initiated by the eNB in response to a request, as part of the X2 Application Protocol (X2AP) for communication between eNBs and gNBs. The primary direction for this message is from the eNB to the gNB, ensuring seamless exchange of UE radio capabilities during network operations.