The correct answer is C) center of gravity will move upwards. When cargo is shifted from the lower hold to the main deck, the center of gravity of the vessel moves upwards. This is because the cargo is being moved from a lower position to a higher position on the ship, causing the overall center of gravity to shift upwards. This change in the vessel's center of gravity can affect the vessel's stability and must be carefully considered when loading and unloading cargo. The other options are incorrect because shifting cargo upwards does not increase the GM (metacentric height) of the vessel, and the center of buoyancy would not move downward, as it is primarily dependent on the shape of the vessel's hull and the displacement of the vessel.