Question: What action does an S-CSCF take regarding the registration-flag for an IMPU at the HSS when it doesn't receive a response to an authentication challenge and the IMPU was not previously registered?
Option 1: The S-CSCF sends a Cx-Put to set the registration-flag to 'Not registered' or 'Unregistered'.
Option 2: The S-CSCF sends a Cx-Put to set the registration-flag to 'Registered'.
Option 3: The S-CSCF does not change the registration-flag at the HSS.
Option 4: The S-CSCF deletes all IMPU-related information from the HSS.
Explanation: According to the specified behavior for handling incomplete authentication, if the IMPU was not already registered and the S-CSCF doesn't receive a response to an authentication challenge, it explicitly updates the registration status at the HSS to reflect the unsuccessful registration attempt. This ensures the HSS accurately reflects the current registration state of the IMPU, which is crucial for subsequent access control and service provisioning decisions.