The correct answer is D) reduce free surface where possible. When a vessel is taking on water but not in immediate danger of sinking, reducing free surface is the best action to increase the vessel's stability. Free surface refers to the movement of liquids within a compartment, which can adversely affect a vessel's stability. By reducing free surface, such as by partially filling tanks or compartments, the vessel's righting ability is improved, increasing overall stability. The other options are incorrect because: A) shifting cargo to the high side would only counter the list, not improve overall stability; B) pumping out double bottoms would reduce draft but not necessarily improve stability; and C) increasing the center of gravity would actually decrease the vessel's stability.