The correct answer is A) Backstay. The backstay is considered standing rigging, not running rigging. Standing rigging refers to the fixed ropes, wires, and cables that provide the permanent structural support for a sailing vessel's mast and boom. Running rigging, on the other hand, refers to the movable ropes and lines used to control the sails, such as sheets, halyards, and downhauls. The other options B) Sheet, C) Downhaul, and D) Halyard are all examples of running rigging, which are used to control the movement and positioning of the sails, whereas the backstay is a part of the standing rigging that provides stability and support to the mast.