The correct answer is C) a portion of liquid is removed from a full tank. The adverse effects due to free surface occur when there is a liquid that is free to move within a tank or compartment on a vessel. When a portion of the liquid is removed from a full tank, the remaining liquid is able to shift and create a free surface, which can have adverse effects on the vessel's stability and handling. This free surface effect reduces the vessel's ability to resist heeling and rolling, which can make the vessel less stable and more prone to capsizing. The other answer choices are incorrect because they do not directly address the issue of free surface. Increasing the vessel's draft (A) or decreasing it (D) can affect stability, but not specifically due to free surface. Trimming the vessel by the stern (B) can also impact stability, but again, not directly due to free surface.