Explanation: Bridge-to-bridge communications, primarily conducted on VHF Channel 13, are specifically mandated by maritime regulations (such as FCC Part 80) for the exchange of vital information directly between vessel bridges, or between a vessel's bridge and a shore station like a Vessel Traffic Service (VTS). The fundamental purpose is **navigational communications** (C) to ensure the safe movement of vessels. This includes discussing passing arrangements, maneuvering intentions, anchoring plans, and any other information necessary to prevent collisions and ensure safe passage in congested waters, narrow channels, or during restricted visibility. Option A is incorrect because while safety is paramount, Channel 16 is the primary distress, urgency, and safety calling channel; bridge-to-bridge is for routine navigational safety to *prevent* emergencies. Options B and D describe internal ship communications, not external communications between vessels or with shore stations.