Answer: Option (D) is the correct answer.
Step-by-step explanation:
Existing model of atom shows that an atom contains neutrons, protons and electrons.
Protons (have positive charge) and neutrons (no charge) reside into the nucleus and nucleus is located at the center of atom.
Whereas electrons revolve around the nucleus. Neutrons are the neutral particles present in an atom as they do not have any charge.
Therefore, out of the given options, an experiment that contradicted an existing model of the atom is described as Chadwick showed that atoms cannot contain neutral particles.