Question: In a network supporting return to the last used E-UTRAN PLMN after CS Fallback, what triggers the Mobility Management Entity (MME) to indicate to the Visitor Location Register (VLR) via the SGs interface?
Option 1: Handover from UTRAN to E-UTRAN
Option 2: UE's initial attachment to the EPS network
Option 3: UE initiating a service request for Mobile Originated (MO) CS Fallback
Option 4: Detachment of UE from the E-UTRAN
Explanation: This is because, as per the described procedure, when a UE initiates a service request for MO CS Fallback in a network configured to return to the last used E-UTRAN PLMN after fallback, the MME uses this specific procedure to notify the VLR. This ensures seamless handling of the UE's request, leveraging the CS Fallback capability as outlined in the context of EPS (Evolved Packet System) for circuit-switched services.