The correct answer is C. MSC.
A mobile switching center (MSC) is a central office in a cellular network that switches calls between mobile phones and the public switched telephone network (PSTN). It also performs billing and system maintenance functions.
A base station is a transceiver that connects mobile phones to the cellular network. It is responsible for receiving and transmitting calls and data.
The PSTN is the traditional telephone network that connects landline phones. It is not responsible for billing or system maintenance in a cellular network.
A mobile system is a general term for a system that allows mobile phones to communicate with each other and with the PSTN. It does not have a specific responsibility for billing or system maintenance.