The cell theory primarily states that all living things are composed of cells, that the cell is the basic unit of life, and that all cells arise from pre-existing cells.
Among the options provided, the statement that best supports the idea that viruses are living is "All living things are composed of cells." However, it's important to clarify that viruses do not fit snugly into this definition because they are not made of cells; rather, they consist of nucleic acid and protein, and they require a host cell to reproduce.
Therefore, while the statement can be interpreted to support discussions about life, viruses are generally not classified as living organisms according to traditional definitions of life. So, the answer reflects a philosophical standpoint rather than a strict adherence to the tenets of the cell theory.