The correct answer is B) High freeboard and sail area. Ro-Ro (Roll-on/Roll-off) vessels are designed to allow wheeled cargo, such as vehicles, to be driven on and off the ship on their own wheels. These vessels are characterized by a high freeboard and large sail area to improve stability and seaworthiness, which allows them to transport vehicles safely across open waters. The other answer choices are incorrect because: A) Long port stays are not a defining characteristic of Ro-Ro vessels, as they are designed for efficient loading and unloading of cargo. C) Ro-Ro vessels typically carry vehicles and cargo, not passengers. D) Lightweight securing equipment is not a unique feature of Ro-Ro vessels, as various cargo ships utilize similar equipment.