Answer: Option (A) is the correct answer.
Step-by-step explanation:
The process in which two or more small nuclei combine together to result in the formation of a larger nuclei is known as fusion.
In Sun, four hydrogen nuclei combine together to result in the formation of helium atom. This combining of small hydrogen nuclei to form a large helium nuclei represents fusion process occurring inside the Sun.
Thus, we can conclude that to power itself, the sun is constantly generating a nuclear reaction in its core, in which hydrogen nuclei are combined to form helium. This process is known as fusion.