Final answer:
Rutherford rejected the "plum pudding model" because his experiments showed atoms are mostly empty space with a tiny, dense nucleus, leading to the nuclear model of the atom where electrons orbit a central nucleus.
Step-by-step explanation:
Rutherford realized that the "plum pudding model" of the atom must not be correct because C) Atoms are mostly empty space with a small, dense nucleus. This conclusion was drawn from Rutherford's gold foil experiment which showed that while most alpha particles passed through the gold foil with little to no deflection, a small fraction was deflected at large angles. These results indicated that the positive charge and most of the atom's mass were concentrated in a tiny central core, not evenly distributed throughout the atom as the plum pudding model suggested. Rutherford's findings led to a new model, where the atom was seen much like a mini solar system with a central nucleus made of protons and neutrons, surrounded by orbiting electrons.
Rutherford's discovery dismantled the plum pudding model proposed by his mentor J. J. Thomson, which depicted the atom as a sphere of positive charge with electrons scattered within, resembling plums in a pudding. Instead, he proposed that an atom consists of a nucleus surrounded by orbiting electrons, shifting the scientific community's understanding of atomic structure towards what is now known as the Rutherford or nuclear model of the atom.