Explanation: The On Scene Coordinator (OSC) is a crucial role in maritime search and rescue (SAR) operations. The OSC is responsible for coordinating the activities of all SAR units *at the scene* of the distress incident. This designation is made by the Search and Rescue Mission Coordinator (SMC), who is typically a shore-based authority such as a Rescue Coordination Center (RCC) or Coast Station. The SMC assesses all available resources and designates the most suitable vessel or aircraft involved in the SAR operation to act as the OSC, based on factors like capability, communication equipment, and crew experience. Therefore, **D) Any involved vessel so designated by the Search and Rescue Mission Coordinator** is the correct answer. The vessel in distress (A) is typically not in a position to coordinate due to its emergency. While the first vessel on scene (B) might be designated, it's not always the case; the SMC makes the determination. Shore authorities (C) generally serve as the overall SMC, not the OSC at the scene.