Question: What action does the GGSN/P-GW take when it has no P-CSCF address available in response to an MS/UE's request for a P-CSCF address in the PCO IE?
Option 1: The GGSN/P-GW sends an error message to the MS/UE.
Option 2: The GGSN/P-GW ignores the request and does not respond.
Option 3: The GGSN/P-GW provides a default P-CSCF address.
Option 4: The GGSN/P-GW responds with an empty PCO IE.
Explanation: According to the text, if the GGSN/P-GW has no P-CSCF address available, it shall ignore the request. This means that the GGSN/P-GW will not send any response or error message to the MS/UE, but rather simply disregard the request.