The correct answer is C) Routine communications can resume after the Rescue Coordination Center transmits a message on the frequency or channel being used for emergency communications stating that such traffic has concluded. This is the correct answer because the Rescue Coordination Center, which is responsible for managing emergency communications, must explicitly authorize the resumption of routine communications after radio silence has been imposed. The other options are incorrect because they do not follow the proper protocol for determining when routine communications can resume - the decision must come from the Rescue Coordination Center, not the vessel's master or an arbitrary determination of frequency usage.