Answer:
a. fusion
Step-by-step explanation:
The Sun's energy is produced at its core through a process that is referred to as nuclear fusion. The type of nuclear fusion involved in the production of energy at the Sun's core is known as Hydrogen fusion.
Hydrogen fusion is a nuclear fusion reaction whereby two atoms of hydrogen form or produce four hydrogen nuclei. These four hydrogen nuclei come together or fuse together to form one helium atom. During this reaction, some mass of hydrogen is changed or converted to light energy in the Sun.