Answer: Helium atoms
Step-by-step explanation: The reaction taking place at the Sun is nuclear fusion reaction. It is a process where two or more atoms combine leading to formation of a new atoms. But here total mass of the reactants is not equal to total mass of the products. Some of the mass is lost as energy and this energy is produced in very enormous amount. At the Sun, four H atoms combine and form helium atoms. Along with this large amount of energy is produced which is responsible for huge amount of heat at the Sun.