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The products of a fusion reaction in the sun's core are characterized by being lighter than the reactants due to the process of nuclear fusion. In fusion, lighter atomic nuclei come together to form a heavier nucleus, and in this process, a small amount of mass is converted into energy according to Einstein's equation, E=mc². This conversion results in products that have less total mass than the starting materials.

For example, in the principal fusion process occurring in the sun, hydrogen nuclei (protons) fuse to form helium nuclei. The mass of the helium produced is less than the total mass of the four protons that combined, with the lost mass released as energy in the form of gamma rays and other forms of radiation, which is what powers the sun and radiates heat and light to our solar system.

The other options do not accurately characterize the products of fusion reactions in the sun's core. The reactants do not produce heavier mass products, nor do they inherently result in free electrons, and the reaction indeed involves the emission of photons, especially as energy is released during the fusion process. Therefore, the characterization of the fusion products being lighter than the reactants aligns with the principles of mass-energy conversion in nuclear reactions.