The correct answer is C) Working frequency. When you know that the vessel you are about to call maintains a radio watch on both the working and calling frequencies, you should call on the working frequency. This is because the working frequency is the frequency designated for routine communications between vessels, while the calling frequency is used to initiate contact. Calling on the working frequency ensures that the vessel is actively monitoring that channel and will be able to receive your transmission. The other options are incorrect because: A) the calling frequency is not the appropriate channel for routine communications; B) the distress frequency is reserved for emergency situations; and D) the urgency frequency is used for urgent, but non-distress, communications.