The main cause of the energy radiation from the Sun is fusion reaction, which is represented by the first option. Fusion reaction is a process in which two or more atomic nuclei are combined to form a heavier nucleus. In the case of the Sun, hydrogen nuclei combine to form helium nuclei. This process releases a tremendous amount of energy in the form of light and heat.

Fission reaction, which is represented by the second option, is the splitting of a heavy nucleus into two or more lighter nuclei. This process also releases energy, but it is not the main cause of energy radiation from the Sun.

Chemical reactions, represented by the third option, involve the rearrangement of atoms to form new substances. While chemical reactions do release energy, they do not occur on the scale or with the intensity required for the energy radiation from the Sun.

Diffusion reaction, represented by the fourth option, is not a specific type of reaction and does not occur on the Sun. Diffusion is a process in which molecules move from an area of higher concentration to an area of lower concentration.

In summary, the fusion reaction is the main cause of the energy radiation from the Sun.