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The correct answer is Option C — Less than the total mass. In nuclear fusion, the product nucleus has slightly less mass than the sum of masses of the individual nuclei. This mass difference called mass defect is converted into energy according to E = mc². This is why nuclear reactions release enormous energy. Option A is incorrect: If mass were equal, no energy would be released because E = mc² with zero mass defect gives zero energy. The energy released in fusion proves mass is lost. Option B is incorrect: Mass cannot increase in fusion. Energy is released not absorbed. If mass increased that would mean the reaction absorbs energy which contradicts observed energy release. Option D is incorrect: Double the total mass is physically impossible as it would require creation of matter from nothing violating conservation of mass-energy. Even a tiny mass defect produces huge energy because c squared is enormously large (9 x 10^16 m^2/s^2).
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