Answer:
B. nuclear fusion
Step-by-step explanation:
Atomic fusion is a process in which two or more lighter nuclei combine to form a heavier nucleus.
The mass of the single nucleus thus formed is lighter than the initial mass.
There is a difference in mass in the heavier nucleus thus formed. According to Einstein's mass-energy equation, the change in mass is equated towards energy.
The energy liberated in the process is called as thermo-nuclear energy.
As in the case of isotopes of hydrogen fuses to form helium, the nuclear reaction is given by relation
₁H³ + ₁H² → ₂He⁴ + ₀n¹ + energy
Thus, the nuclear reaction of hydrogen isotopes forming helium is nuclear fusion.