The major problem in harnessing nuclear energy is how to

Explanation:

• In recent times, production of radioactive wastes such as uranium mill tailings, spent (used) reactor fuel, and other radioactive wastes is the primary environmental hazard associated with nuclear energy. These elements can stay radioactive for a period of thousand years resulting in hazard to human health. 
• The major problem is disposing of the waste generated from nuclear plants because they are highly radioactive in nature. Thus, it is difficult to dispose of spent fuel safely.
• To safeguard human health and the environment, radioactive wastes are subject to particular rules that govern their handling, transportation, storage, and disposal.

Final Answer:

Hence, option C is correct i.e, dispose off spent fuel safely.