What does the cell theory assert about living things?

The assertion that all living things are made up of cells is a fundamental principle of cell theory, which is one of the cornerstones of biology. This theory states that not only are cells the basic unit of life, but also that all living organisms, whether unicellular or multicellular, consist of one or more cells. This understanding emphasizes the role of cells as the building blocks of life, showing that they perform essential functions necessary for the organism's survival.

The other options, while related to biological concepts, do not reflect the main tenets of cell theory. For instance, while it is true that all matter, including living things, is ultimately composed of atoms, that fact goes beyond the scope of cell theory itself. The idea that cells can arise spontaneously from non-living matter is a misconception that has been disproven by experiments, establishing that cells come from the division of preexisting cells. Additionally, stating that all organisms are multicellular is inaccurate, as many living organisms, like bacteria and certain protists, are unicellular. Thus, the focus on cells as the foundational unit of life is the essence of cell theory, making the assertion that all living things are made up of cells the correct interpretation of the question.