The correct answer is C) pelorus. A pelorus is a compass card without north-seeking capability that is used for relative bearings, such as determining the bearing of an object relative to the ship's heading. It allows the user to measure the horizontal angle between the ship's heading and the direction of an object. The other options are incorrect because: A) a bearing bar is a tool used to plot bearings on a chart, B) a bearing circle is a device used to measure the bearing of an object from the ship's heading, and D) an alidade is an instrument used to measure vertical angles, not relative bearings.