Rutherford`s alpha-particle scattering experiment was a crucial experiment in the field of atomic physics that led to the discovery of the nucleus. This experiment involved firing alpha particles (helium nuclei) at a thin gold foil. The expected outcome, according to the Thompson model of the atom at that time, was that the alpha particles would pass through the foil with only slight deflections.

However, the actual results obtained were quite different. Rutherford observed that some alpha particles experienced significant deflections, defying the predictions of the Thompson model. He concluded that there must be a dense, positively charged region within the atom that was responsible for these deflections. This dense region, later named the nucleus, was found to contain protons and neutrons.

Therefore, while options 1 (electron), 2 (proton), and 4 (helium) are all parts of an atom, none of them were directly discovered through Rutherford`s experiment. The correct answer, option 3 (nucleus), accurately identifies the outcome of the experiment and its contribution to our understanding of the atomic structure.