Rutherford`s alpha-particle scattering experiment was responsible for the discovery of the atomic nucleus. During this experiment, Rutherford bombarded a thin sheet of gold foil with alpha particles. According to the prevailing model at that time, the Thomson model, it was believed that atoms were composed of uniformly distributed positive charge with electrons embedded within them, similar to a plum pudding.

However, Rutherford`s experiment showed unexpected results. The majority of alpha particles passed through the gold foil with little or no deflection, which was consistent with the existing model. But to everyone`s surprise, a small fraction of alpha particles were deflected at large angles, and in some cases, even bounced back. This observation led Rutherford to conclude that atoms were not uniformly distributed positive charge, but rather contained a small, dense, and positively charged region in the center called the atomic nucleus.

Therefore, option 3, "Atomic Nucleus", is the correct answer because Rutherford`s experiment revealed the presence and characteristics of the atomic nucleus within the atom.