The correct answer is B) in the geometric center of the triangle. When you plot a fix using three lines of position and they intersect in a triangle, the vessel's position should be plotted at the geometric center of that triangle. This is because the triangle represents the area of uncertainty, and the geometric center is the most likely position of the vessel based on the intersecting lines of position. The other options are incorrect because: A) is too specific and does not account for the full triangle of uncertainty, C) is too broad and could place the vessel anywhere within the triangle, and D) is incorrect as the vessel should be located within the triangle, not outside of it.