(D)
Step-by-step explanation:
Why (A), (B) and (C) don't make sense:
(A) you can't get high energies by breaking low-energy bonds. Simple law of energy conservation at work prevents it from happening.
(B) & (C) adding protons to the nucleus is not feasible due to the natural repulsion between protons. By the same token, protons cannot be removed from the nucleus by external means because of the incredibly powerful nuclear forces that bind the protons and neutrons together.
Why (D) is the correct answer:
It takes a lot of energy to hold the nucleus together. But by introducing neutrons into the nucleus, the nucleus can be rendered unstable and become radioactive. So when the nucleus breaks up, the energy that used to hold it together is now released and this energy can be harnessed to generate power.