Which foil was used in the alpha-particle scattering experiment? 

Gold 

The correct answer is option 3. Gold.

The foil used in the alpha-particle scattering experiment, commonly known as the Rutherford gold foil experiment, was Gold.

The alpha-particle scattering experiment was conducted by Ernest Rutherford and his colleagues in 1909. In this experiment, alpha particles (helium nuclei) were directed at a thin foil of gold. The purpose of the experiment was to investigate the structure of the atom and the distribution of charge within it.

Gold foil was chosen for this experiment because it could be made extremely thin, allowing alpha particles to pass through it with minimal obstruction. Additionally, gold is highly malleable, enabling the production of foils that are only a few atoms thick.

The surprising result of the experiment was that while most alpha particles passed through the gold foil with little deflection, some were scattered at large angles, and a few even bounced back. This led Rutherford to propose a model of the atom where most of the atom's mass and positive charge were concentrated in a small, dense nucleus, with electrons orbiting around it.

Therefore, gold foil was specifically chosen for the alpha-particle scattering experiment due to its properties, making it the correct answer.