The answer is D. This is Rutherford’s experiment where he wanted to prove the existence of a nucleus in an atom. Ernest Rutherford along with his students Ernest Marsden and Hans Geiger bombarded alpha particles into a very thin gold foil and had an observation that 1/8000 of the particles deflated at a large angle.

They observed it under a microscope on a screen of zinc sulfide in the dark. It was then they confirmed that strong electric fields exist. He then concluded that a positive-charged atom lies in a compact and solid nucleus. This nucleus concentrates almost the whole mass of the atom but occupies only hundred millionth of a millionth of the atom’s volume.