In a nuclear fusion reaction, the total mass numbers of the reactants may equal the total mass numbers of the products. How can it still be true that mass is lost in nuclear fusion reactions?
In a nuclear fusion reaction, the fused nucleus is initially stable but then decays and loses mass.
The mass number is just an estimated mass measurement that is not precise.
The mass number only represents the total number of nucleons, which may be conserved.
In a nuclear fusion reaction, the products have greater kinetic energy than the reactants.

Question

In a nuclear fusion reaction, the total mass numbers of the reactants may equal the total mass numbers of the products. How can it still be true that mass is lost in nuclear fusion reactions? 
    In a nuclear fusion reaction, the fused nucleus is initially stable but then decays and loses mass. 
   The mass number is just an estimated mass measurement that is not precise. 
   The mass number only represents the total number of nucleons, which may be conserved. 
   In a nuclear fusion reaction, the products have greater kinetic energy than the reactants.

anonymous · Answer

The third choice is correct.  "Mass number" is just the count of nucleons, the actual mass differs, in part because the mass of a free nucleon is not exactly 1 in amu (a free proton, for example, has a mass of 1.007276 amu), and in part because nucleons lose mass when they combine to form a nucleus, and the amount of this "mass deficit" varies from nucleus to nucleus.