The correct answer is C) sway. Sway is the side-to-side motion of a vessel, which can significantly increase the tension on mooring lines. This is because sway causes the vessel to move laterally, putting additional strain on the mooring lines. The other motions listed - yaw, roll, and pitch - primarily involve rotational movements that are less likely to directly impact mooring line tension. Yaw is the side-to-side rotation of the vessel, roll is the side-to-side tilting motion, and pitch is the forward and backward rocking motion. While these motions can indirectly affect mooring line tension, sway is the primary culprit in significantly increasing the tension on the lines.