Explanation: The INVITE message is used to initiate a call setup process between two user equipment devices. This is evident from the sequence diagram, where UE-1 sends an INVITE message to P-CSCF, which then forwards it to S-CSCF and finally reaches AS (Application Server). The subsequent messages, such as ACK and 200 OK, confirm the successful establishment of the call session. Therefore, Option 1 is the correct answer.