The correct answer is fusion.
For a nuclear fusion to occur in the sun it takes high temperature.
Nuclear fusion is termed as reaction whereby two or more atomic nuclei they come together to form subatomic particles and different atomic nuclei.
The release of large amounts of energy is manifested in the different in mass which is between products and reactants. Helium and hydrogen are among the lighter elements which are fusible.
Plutonium, thorium, uranium are heavier elements and are fissionable.