Answer: A. Gold foil experiment
Step-by-step explanation:
Ernest Rutherford discovered the nucleus in 1911 from the results he obtained in several experiments in which he bombarded a thin sheet of gold with alpha particles (a sub-microscopic particle with a positive charge) from a radioactive element.
Rutherford observed, by means of a fluorescent screen, to what extent the particles with which he bombarded the sheet were scattered. Most of them traversed the metal sheet without changing direction; however, a few were reflected backwards with small angles.
Using a mathematical analysis of the forces involved, Rutherford showed that the scattering was caused by a small positively charged nucleus, located in the center of the gold atom. In this way, he deduced that most of the atom is empty space and discovered the atomic nucleus.