What is defined as the smallest entity of matter that contains subatomic particles?

The smallest entity of matter that contains subatomic particles is defined as an atom. Atoms are the fundamental building blocks of matter and consist of a nucleus that contains protons and neutrons, surrounded by electrons that orbit the nucleus. This structure enables atoms to interact and combine in various chemical processes, forming molecules and compounds.

Molecules are formed when two or more atoms bond together, while compounds are specific types of molecules that contain atoms of different elements. An element is a pure substance that cannot be broken down into simpler substances by chemical means and is made up of only one type of atom. Thus, while all these terms are related to the structure of matter, the atom is the foundational unit that houses subatomic particles, making it the correct answer in this context.