What does atomic theory propose about the composition of matter?

Atomic theory proposes that matter is composed of atoms, which are the fundamental building blocks of all substances. This idea suggests that all matter, whether solid, liquid, or gas, can be broken down into these tiny indivisible units called atoms. Each atom can combine with others to form molecules, but the core principle of atomic theory emphasizes that atoms are the basic unit that makes up all forms of matter.

The other choices present important aspects of science but do not accurately represent the fundamental premise of atomic theory. For example, while it is true that some matter is composed of molecules, this is a broader category that includes atoms as well. Similarly, cells are the basic units of life in biology, but they do not apply to all matter in the physical sciences. The statement about elements is related to the classification of matter in terms of pure substances but does not capture the essential idea of atomic composition proposed by atomic theory.