Question: What triggers the increment of the 'Number of check IMEI requests' measurement in a UMTS network?
Option 1: Receipt of a 'MAP_CHECK_IMEI' response from the EIR with a white-listed equipment status.
Option 2: Transmission of a 'MAP_CHECK_IMEI' request to the Equipment Identity Register (EIR).
Option 3: A handover procedure between two SGSNs in a packet-switching domain.
Option 4: Detection of an unknown IMEI by the User Equipment (UE).
Explanation: The 'Number of check IMEI requests' measurement increments upon transmitting a 'MAP_CHECK_IMEI' request to the EIR, aligning with UMTS Performance Management specifications for monitoring network element interactions (e.g., SGSN and EIR) during equipment identity checks.