The correct answer is B) upward-acting buoyant force. Stability is determined principally by the location of the center of gravity and the center of buoyancy. The downward-acting gravity force acts at the center of gravity, while the upward-acting buoyant force acts at the center of buoyancy. The relationship between these two points determines the vessel's stability. If the center of buoyancy is above the center of gravity, the vessel will be stable. Therefore, the upward-acting buoyant force is the key factor in determining a vessel's stability. The other options are incorrect because the downward-acting weight force (A) is the same as the downward-acting gravity force, and the upward-acting weight force (C) does not exist. Environmental forces (D) can affect stability, but they are not the principal determinants.