The correct answer is B) sway. Sway refers to the horizontal port or starboard movement of a vessel. This is one of the six degrees of freedom that describe the motion of a vessel, the others being surge (forward and backward movement), heave (up and down movement), roll (rotation around the longitudinal axis), pitch (rotation around the transverse axis), and yaw (rotation around the vertical axis). The other answer choices are incorrect because heave refers to the up and down movement, surge refers to the forward and backward movement, and yaw refers to the rotation around the vertical axis, rather than the horizontal port or starboard movement.