Concept:Percentage by mass of an element in a compound is calculated by dividing the total mass of that element by the molar mass of the compound, then multiplying by 100.Explanation:In MgSO4⋅7H2O, oxygen atoms are present in both parts.MgSO4 contains 4 oxygen atoms.7H2O contains 7 oxygen atoms.Total oxygen atoms =4+7=11.Atomic mass of oxygen =16.Total mass of oxygen =11×16=176.Given molar mass of the compound =246.Percentage of oxygen =246176×100.Percentage of oxygen =71.544%≈71.5%.Answer:C. 71.5%