The correct answer is C) Yaw. Yaw is the motion of a vessel that describes its rotation around a vertical axis, which affects the vessel's directional stability. Directional stability is the ability of a vessel to maintain a straight course when subjected to external forces such as wind and waves. Yaw motion directly impacts the vessel's ability to maintain its heading, making it the relevant motion that affects directional stability. The other options, such as pitch (rotation around the transverse axis) and sway (sideways movement), do not directly influence the vessel's directional stability in the same way that yaw does. Surge (forward and backward movement) also does not impact directional stability.