Rutherford's scattering reveals a dense nucleus

Steps:

• Alpha particles were fired at thin gold foil to probe atomic structure.
• Most particles passed straight through, indicating atoms are mostly empty space.
• Rare large-angle deflections suggested a small, massive, positively charged core repelling alphas.
• This core is the nucleus, as explained by Rutherford's model.

Why A is correct:

• Large-angle deflections of few alphas indicate close encounters with a tiny, dense positive charge (nucleus), per Coulomb's law of electrostatic repulsion.

Why the others are wrong:

• B: Unchanged energy shows elastic scattering but not the cause (nucleus).
• C: Slower particles deflect more due to longer interaction time, unrelated to nucleus existence.
• D: Thickness affects scattering count via more atoms, but doesn't prove a nuclear structure.

Final answer: A