Explanation: The INVITE message is used to initiate a call setup, but the correct answer is related to acknowledging receipt of the request and readiness to receive media. This is evident in the sequence where UE-1 sends an INVITE (step 1) and P-CSCF responds with a 100 Trying response (step 2), indicating it has received the INVITE and will process it further. The correct option reflects this understanding of the call flow protocol.