J. J. Thomson's plum-pudding model pictured the atom as a sphere of uniformly spread positive charge in which the tiny negative electrons were embedded like plums in a pudding. A key prediction of this model is that, because the positive charge and mass are spread thinly and evenly, a fast, positively charged particle fired at the atom should pass through with only very small deflections.

This prediction was tested in the Geiger–Marsden experiment (the gold-foil experiment), carried out under Ernest Rutherford. A thin gold foil was bombarded with high-speed alpha particles, and the following was observed:

• The great majority of alpha particles passed straight through the foil, showing that the atom is mostly empty space.
• A small number were deflected through large angles, and a very few (about 1 in 20,000) even bounced almost straight back.

The large-angle deflections were impossible to explain with Thomson's thinly spread charge, which could never exert a force strong enough to turn an alpha particle sharply. The only way to account for such violent scattering was to assume that almost all of the atom's mass and its entire positive charge are packed into an extremely small, dense central region — the nucleus. This is why the observation of alpha-particle scattering showing deflection at large angles is the correct answer.

The other options concern later or unrelated developments. The line spectra of hydrogen led Bohr to quantise electron orbits, but that refined Rutherford's model rather than directly overthrowing Thomson's. X-ray diffraction measures atomic spacings in crystals, not the internal charge distribution of an atom. Chadwick's discovery of the neutron came much later and explained atomic mass and isotopes; it did not disprove the plum-pudding model. Only the gold-foil scattering result directly contradicted and replaced Thomson's model.