Which concept states that all things are made up of atoms?

The Atomic Theory of Matter is the correct choice because it fundamentally posits that all matter is composed of atoms, which are the basic units of chemical elements. This theory provides a comprehensive explanation of the nature and behavior of matter, suggesting that atoms combine in various ways to form molecules and compounds. The theory has evolved over time, starting with early concepts proposed by ancient philosophers and later formalized through scientific investigations in the 19th century, particularly by scientists such as John Dalton.

In contrast, quantum theory primarily focuses on the behavior of particles at atomic and subatomic levels, rather than specifically stating that all matter is made up of atoms. Relativity theory, formulated by Albert Einstein, addresses the laws of physics in relation to observers in different frames of reference and does not concern itself with the composition of matter at the atomic level. Cell theory is a fundamental concept in biology that outlines the properties and functions of cells, asserting that all living organisms are composed of cells and that the cell is the basic unit of life. However, it doesn't encompass non-living matter or provide insight into the atomic composition of all things. Thus, the Atomic Theory of Matter is the most appropriate and encompassing answer in this context.