What happens during the entire fusion process in the sun? two hydrogen nuclei come together to produce one helium nucleus and two protons, and energy is released. four hydrogen nuclei come together to produce one helium nucleus and two protons, and energy is released. six hydrogen nuclei come together to produce one helium nucleus and two protons, and energy is released. eight hydrogen nuclei come together to produce one helium nucleus and two protons, and energy is released.

Two hydrogen nuclei come together to produce one helium nucleus and two protons, and energy is released.

Explanation:

All stars (including the Sun) at birth begin to fuse two hydrogen nuclei to form helium through nuclear fusion.  

In this sense, nuclear fusion is the union of two light atoms, releasing energy in the process, which is the cause of the great luminosity of a star. Especifically, the hydrogen nuclei will begin to fuse, generating helium nuclei in the process and releasing huge amounts of energy.

To unerstand it better:

To shine, the stars transform their hydrogen into helium by means of nuclear fusion. When at half of its life a star is without hydrogen, the nucleus becomes entirely helium and the star declines, becoming colder and brighter due to the energy generated by the nuclear reactions, then the star begins to contract.  

Nevertheless, if the star is bigger, this helium will be also consumed and the nucleus transformed in Carbon, then in Oxigen, and so on. Being the last transformation Iron until the star delivers all its energy.