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The correct answer is Option C — Less than the total mass. 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. 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).
