The correct answer is D) make the vessel stiffer. Shifting a weight from the upper 'tween deck to the lower hold will decrease the vessel's metacentric height, which in turn will make the vessel stiffer. This is because lowering the center of gravity of the vessel increases its initial stability, making it more resistant to rolling motions. Conversely, the other options are incorrect - increasing the rolling period, decreasing the metacentric height, and making the vessel more tender are all effects that would result from moving the weight to a higher position, not a lower one.