Which description of the expected behavior of a SIP User Agent Client that received a reliable provisional response that contains an offer is true?
A. The UAC must include an answer in a PRACK
B. The UAC must include an answer in a PRACKACK
C. The UAC may include an answer in PRACKACK
D. The UAC may include an answer in PRACK
E. The UAC may include an answer in ACK
A – PRACK method contianing a RAck header
Upon receipt of that response message, the SIP UAC will reply with a Prack message containing a Response Acknowledge (RAck) header with the same RSeq value.
The following message flow is an example of a Prack transaction.
UAC sends: INVITE (Supported: 100Rel)
UAS sends: 180 Ringing (Required: 100Rel, RSeq: 145)
UAC sends: PRACK (CSeq: 1 PRACK, RAck: 145)
UAS sends: 200 Ok (CSeq: 1 PRACK)
I believe the answer is A
If the UAC receives a reliable provisional response with an offer (this would occur if the UAC sent an INVITE without an offer, in which case the first reliable provisional response will contain the offer), it MUST generate an answer in the PRACK.
https://www.ietf.org/rfc/rfc3262.txt