In which experiment did Ernest Rutherford discover the presence of a dense, positively charged nucleus?

The discovery of a dense, positively charged nucleus was made during the gold foil experiment conducted by Ernest Rutherford in 1909. In this experiment, alpha particles were directed at a thin foil of gold. Rutherford observed that while most of the alpha particles passed through the foil with little or no deflection, a small number were deflected at very large angles, and some even bounced back toward the source.

This unexpected result led Rutherford to conclude that the majority of the atom must be empty space, with a very small, dense, and positively charged center, which he referred to as the nucleus. This was a significant shift from the previous plum pudding model proposed by J.J. Thomson, which suggested that positive and negative charges were distributed uniformly throughout the atom. Rutherford's findings laid the groundwork for the modern understanding of atomic structure, establishing that the nucleus is compact and contains most of the atom's mass.

The other experiments listed do not deal with atomic structure in the same way. The cathode ray tube experiment investigated the properties of electrons, the oil drop experiment measured the charge of the electron, and the electrolysis experiment focuses on chemical reactions in solutions, rather than the internal structure of the atom.