The correct answer is B) Backstay. The backstay is part of a vessel's standing rigging, which refers to the fixed rigging that supports the masts and keeps them in position. The backstay is a vertical or diagonal rope or cable that extends from the top of the mast to the deck or stern of the vessel, providing crucial support and stability to the mast. The other options are incorrect because: A) Sheet is a line used to control the angle of a sail. C) Downhaul is a line used to pull down a sail. D) Topping lift is a line used to support the boom.