Rutherford`s alpha particle scattering experiment on thin gold foil was responsible for the discovery of the atomic nucleus.

In this experiment, Rutherford fired alpha particles (positively charged particles) at a thin sheet of gold foil. According to the prevailing theory at the time, the "plum pudding" model, it was believed that atoms were made up of a positively charged "pudding" with negatively charged electrons scattered throughout.

However, Rutherford`s observations were unexpected and groundbreaking. Most of the alpha particles passed straight through the gold foil with minimal deflection, but a small fraction of the alpha particles were deflected at large angles, and a few even bounced straight back. This result could not be explained by the plum pudding model.

Rutherford concluded that the atom must have a tiny, dense, positively charged core at its center, which he called the atomic nucleus. This nucleus contains almost all of an atom`s mass and is surrounded by electrons. The discovery of the atomic nucleus revolutionized our understanding of atomic structure and led to the development of the modern model of the atom.

Overall, Rutherford`s alpha particle scattering experiment provided strong evidence for the existence of the atomic nucleus, making option 3, "atomic nucleus," the correct answer.