Question: What cipher key is used to encrypt signalling radio bearers for services delivered by both CS and PS service domains?
Option 1: A combination of CKCS and CKPS
Option 2: The CK of the service domain for which the most recent security mode negotiation took place
Option 3: A separate signalling cipher key (CKSIG)
Option 4: Either CKCS or CKPS, depending on the type of user data being transferred
Explanation: According to the text, the signalling radio bearers are ciphered by the CK of the service domain for which the most recent security mode negotiation took place. This means that the same cipher key used for encrypting user data in a particular service domain is also used for encrypting signalling data related to that service domain.