The main source of energy in the Sun is nuclear fusion. Nuclear fusion is a process in which atomic nuclei combine to form a heavier nucleus, releasing a large amount of energy in the process. In the Sun, this fusion process primarily involves hydrogen nuclei combining to form helium.

Option 2, nuclear fission, is not the main source of energy in the Sun. Nuclear fission is the process in which an atomic nucleus is split into two or more smaller nuclei, and while it is used as a source of energy in nuclear power plants, it is not the primary mechanism by which the Sun generates its energy.

Option 3, chemical reaction, is also not the main source of energy in the Sun. Chemical reactions involve the rearrangement of atoms to form new molecules, and they do not release the same amount of energy as nuclear reactions.

Option 4, mechanical energy, is not the source of energy in the Sun either. Mechanical energy is the energy associated with the motion and position of objects, and it is not the primary mechanism by which the Sun produces its energy.

Therefore, the correct answer is option 1, nuclear fusion, as it accurately represents the main source of energy in the Sun.