Nuclear energy is generated by a process called nuclear fission, which involves splitting the nucleus of an atom. This option explains that nuclear fission is the method responsible for generating nuclear energy. However, the statement that its expression was proposed by Einstein is not accurate. The concept of nuclear fission was actually proposed by Otto Hahn and Fritz Strassmann, while the underlying principles were explained by Lise Meitner and Otto Frisch, who discovered that the nucleus of uranium could be split into smaller fragments.

To clarify, Albert Einstein did contribute to the field of nuclear physics with his theory of relativity, but he did not propose nuclear fission. It is important to understand that nuclear fission is a result of nuclear reactions and the release of energy from the splitting of atomic nuclei, rather than being a direct expression or proposal by any specific scientist or physicist.

Therefore, the correct answer is option 1, but it should be noted that the statement regarding Einstein`s proposal is incorrect.