The correct answer is C) The GM will increase. When cargo is shifted from the main deck into the lower hold of a vessel, the center of gravity of the vessel will be lowered, resulting in an increase in the vessel's GM (metacentric height). This is because the weight of the cargo is moved closer to the vessel's baseline, which reduces the vessel's tendency to roll and increases its stability. The metacenter and center of buoyancy do not necessarily move upward in this scenario. The other answer choices are incorrect because: A) The metacenter may not necessarily move upward. B) The center of buoyancy may not necessarily move upward. D) While all the changes mentioned in the other choices can occur, the key change is the increase in GM.