The correct answer is C) A preventer. A preventer is used to prevent the boom from accidentally jibing when running before the wind in a fresh breeze. A preventer is a line that is attached to the boom and led forward to a strong point on the vessel, providing additional support and control over the boom's movement. This helps to prevent the dangerous and sudden swing of the boom that can occur when running before the wind, which could potentially cause injury or damage. The other answer choices are incorrect because they do not directly address the issue of preventing an accidental jibe. A buntline, outhaul, and clewline serve different purposes related to sail handling and adjustment, but do not specifically prevent the boom from swinging uncontrollably.