Solution:

• Hydrogen as a fuel in Sun. The source of Sun’s incessant energy is hydrogen which is continuously converting to helium through nuclear fusion reaction releasing energy.
• Nuclear fusion is the type of nuclear reaction taking place in the sun.In nuclear fusion, two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles.
• Here the hydrogen atoms combined to form helium and heat is liberated during this process,hence hydrogen is known as nuclear fuel in the sun.