The correct answer is A) bring the vessel's head into the wind and let the sails luff. Bringing the vessel's head into the wind and letting the sails luff is the most effective way to slow or stop a sailing vessel. When the sails luff, they lose their ability to generate forward thrust, causing the vessel to slow down or stop. This is the standard technique used to control the speed of a sailing vessel. The other options are incorrect because they do not effectively slow or stop the vessel. Raising the centerboard (B) and putting the wind off the beam or stern (C, D) may change the vessel's direction, but they do not necessarily cause the vessel to slow down or stop.