What are the tiny, unseeable parts that make up all matter called?

The term that encompasses the tiny, unseeable parts making up all matter is best described as atoms. Atoms are the fundamental building blocks of matter, consisting of protons, neutrons, and electrons. They combine to form molecules, which in turn make up different substances that we can observe. While the term "particles" can refer to various subatomic particles or even larger collections of atoms, it is less specific than "atoms" as a designation for the basic unit of matter.

Molecules are groups of two or more atoms bonded together, which are larger entities than individual atoms. Cells refer to biological units of life, comprising various molecules and complexes, and are not directly associated with the basic structure of matter in its simplest form. Thus, atoms accurately represent the foundational units of matter, making it the correct answer.