Answer:
B. An atom is a solid sphere
Explanation:
The idea that the atom was a solid sphere paved back to the first atomic model in the 19th century theorized by John Dalton. According to him, an atom is intact and hard and cannot be broken down or divided.
However, many scientists continued to research and it was found that the atom was not actually a solid sphere. It was made up of protons (positively-charged), electrons (negatively-charged) and neutrons (neutrally-charged). This invalidated Dalton's solid sphere model.