In nuclear fusion, two light atomic nuclei combine to form a heavier nucleus. During this process, the mass of the resulting nucleus is typically less than the sum of the masses of the original reactants due to the release of energy, as described by Einstein's equation \(E=mc^2\). This energy release is associated with the mass defect.
Thus, the correct response is:
A nucleus with a smaller mass than any of the reactants will be produced.