The correct answer is A) displacement volume of the vessel. The free surface effects of a partially full liquid tank decrease with increased displacement volume of the vessel. This is because as the overall displacement of the vessel increases, the relative effect of the free surface in the partially full tank becomes less significant. The larger the vessel, the smaller the impact of the free surface on the vessel's stability and overall performance. The other options are incorrect because they do not directly affect the free surface effects. The placement of the tank above the keel (B), the size of the surface area in the tank (C), and the density of the liquid (D) do not have a direct relationship with the decrease in free surface effects as the vessel's displacement volume increases.