The correct answer is B) Bow spring line. The bow spring line is the most useful line when maneuvering a vessel alongside a pier without tug assistance. The bow spring line runs from the bow of the vessel to the pier and, when tightened, helps control the movement of the bow and keep it close to the pier during the docking maneuver. This allows the vessel to be positioned accurately without relying on other lines. The other options are less effective in this scenario. The inshore head line (A) and stern breast line (C) are not as crucial for controlling the bow during the docking process. The bow breast line (D) could be useful but is secondary to the bow spring line in terms of its importance for this particular maneuver.