Which statement best summarizes the cell theory?

The statement that cells are the smallest units of life and come from pre-existing cells encapsulates the core principles of cell theory in biology. This theory is fundamental because it establishes that all living organisms are composed of cells, marking them as the basic building blocks of life. Furthermore, the idea that cells arise from pre-existing cells underscores an essential biological process – cell division – which is crucial for growth, repair, and reproduction in living organisms. This principle negates the notion of spontaneous generation, where cells or living organisms are thought to arise from non-living matter.

The other statements do not accurately represent the cell theory's foundational concepts. For instance, saying that all cells are identical in structure and function is inaccurate, as there is significant diversity among cells, reflecting their specialized functions within different organisms. The assertion that all life forms are unicellular organisms ignores the existence of multicellular organisms, which also play a vital role in the biosphere. Lastly, the idea that cells can arise from non-living matter contradicts a key aspect of cell theory which emphasizes the necessity of existing cells for the creation of new cells. This highlights the correctness of the chosen statement and its alignment with established scientific understanding.