The correct answer is D. 51.2%. Magnesium sulfate heptahydrate, MgSO4•7H2O, is a compound that contains magnesium, sulfur, oxygen, and hydrogen. The percent composition of water in this compound can be calculated by determining the mass of the water molecules and dividing it by the total mass of the compound. The mass of the water molecules is 7 x (2 x 1.008) = 14.56 g. The total mass of the compound is 246.4 g. Therefore, the percent composition of water in the compound is 14.56/246.4 x 100 = 51.2%.