The correct answer is B) shrouds. Shrouds are the sets of ropes or wires that run from the mast to the sides of the boat, helping to support the mast and prevent it from moving sideways. The shrouds resist the lateral or sideways forces acting on the mast, keeping it stable and upright. The other options are incorrect because: A) Sheets are lines used to control the sails, not the mast. C) Halyards are used to raise and lower the sails, not to stabilize the mast. D) The forestay is a cable that runs from the bow to the top of the mast, supporting it from the front, not the sides.