What process combines lighter elements to produce heavier elements, and occurs in the core of the sun and other stars, producing ultraviolet and visible light?

The correct answer is a fusion reaction.  

In the process, the energy is discharged when two light atomic nuclei are combined together to produce one heavier atom. This procedure provides the energy that powers the Sun and other stars, in the process combination of hydrogen nuclei takes place to generate helium.  

In order to attain high fusion reaction rates, that is, to produce fusion as a beneficial source of energy, the two kinds of hydrogen, that is, tritium and deuterium need to be heated over the temperatures of 100 million degrees Celsius, and at these temperatures, the fuel turns into a plasma.