Which process generates the energy that is released by stars?
(1) nuclear fusion (3) convection currents
(2) thermal conduction (4) radioactive decay

Nuclear fusion generates the energy that is released by stars. 
In the great majority of stars, it's the fusion of hydrogen nuclei
to form helium nuclei plus energy.  In very old stars, the nuclei
of heavier elements are fused, forming even heavier ones.

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