The correct answer is B) Concentrate weights on upper decks. Concentrating weights on the upper decks of a stiff vessel will raise the metacentric height (GM), which increases the vessel's natural roll period and results in a more comfortable roll. This technique increases the vessel's stability and reduces its tendency to make sudden, jarring rolls. The other options are incorrect because: A) Ballasting the peak tanks would not significantly affect the roll characteristics; C) Adding weight near the centerline of the lower hold would lower the GM and make the vessel even stiffer; and D) Moving weights lower in the ship would also lower the GM and exacerbate the stiff rolling characteristics.