The correct answer is B. The reduction in GM (metacentric height) due to free surface effect is calculated using the formula: Reduction in GM = I / (W * GM) Where: I = Moment of Inertia of the free surface W = Displacement of the vessel GM = Metacentric Height Given: Displacement = 645 tons Length = 132 ft Beam = 34 ft The moment of inertia (I) of a rectangular free surface is calculated as: I = L^3 * B / 12 Plugging in the values: I = 132^3 * 34 / 12 = 1,280,832 ft^4 Applying the formula: Reduction in GM = 1,280,832 / (645 * GM) This is the correct expression for calculating the reduction in GM due to free surface effect.