The correct answer is C) Lubber's line. The lubber's line is a fixed mark on the compass that indicates the heading of the vessel. It aligns with the bow (front) of the ship, allowing the navigator to read the vessel's heading directly from the compass card. The compass card itself is not a fixed part of the compass, but rather a rotating element that indicates the cardinal directions. The needle and gimbals are also part of the compass, but they do not directly indicate the vessel's heading. Therefore, the lubber's line is the correct part of the compass that shows the heading of the vessel.