Question: In a UMTS network, what triggers the counting of 'Attempted relocation preparations with UE involved for CS domain'?
Option 1: Transmission of a RANAP message RELOCATION REQUIRED from the source RNC to the CS CN with Relocation Type set to 'UE involved in relocation of SRNS'
Option 2: Receipt of a UE-originated CS call setup request at the target RNC
Option 3: Successful handover of a UE from the source RNC to the target RNC for PS domain
Option 4: Periodic location update by a UE in idle mode
Explanation: This is correct because the transmission of a specific RANAP message - RELOCATION REQUIRED with Relocation Type set to 'UE involved in relocation of SRNS' - from the source RNC to the CS CN, marks an attempted relocation preparation involving the UE, directly aligning with the definition of the measurement. The other options do not match this specific trigger.