Answer: empty space.
- A empty space
- B solid matter
- C a liquid
- D a dense gas
Correct answer: A. empty space
Explanation: The nucleus is minute compared with the atom, so an atom is mostly empty space.

The Geiger–Marsden alpha-scattering result: a few alpha particles bounce back at large angles, which rules out Thomson’s diffuse model and demands a tiny, massive, positively charged nucleus. Image: Kurzon, CC BY 3.0, via Wikimedia Commons.
Concept context
Thomson's plum-pudding model, Rutherford's nuclear model from alpha-scattering, Bohr's postulates and hydrogen spectrum, spectral series (Lyman to Pfund), Rydberg formula, and limitations of each model.