Question: Under what condition(s) does the P-CSCF attach an indication to a REGISTER request informing the S-CSCF that the message was **not** integrity protected?
Option 1: When the REGISTER request contains an authentication response and is protected with an SA created during this authentication procedure.
Option 2: When the REGISTER request does not contain an authentication response but is protected with an SA from the latest successful authentication.
Option 3: For all REGISTER requests that do not meet either of the specified conditions for integrity protection (e.g., no authentication response and no valid SA, or scenarios outside the two defined cases).
Option 4: Never; the P-CSCF always assumes integrity protection for REGISTER requests.
Explanation: The correct answer is Option 3 because the P-CSCF attaches an indication of no integrity protection for all scenarios not meeting the specified conditions (e.g., no authentication response and no valid Security Association), thus informing the S-CSCF the REGISTER request was **not** integrity protected.