The correct answer is D) 9.27 feet. The reduction in GM (metacentric height) due to free surface effect is calculated using the formula: GM reduction = (L*B*h^3)/(12*V), where L is the length of the flooded space, B is the breadth of the flooded space, h is the height of the water, and V is the vessel's displacement. In this case, the flooded space is the 65'L x 38'B after deck, the height of the water is the weight of the water (76 tons) divided by the water density and the area of the flooded space, and the vessel's displacement is 840 tons. Plugging these values into the formula gives the reduction in GM as 9.27 feet. The other options are incorrect because they do not accurately reflect the reduction in GM based on the given information.