The cell theory includes that D) All living things are made up of cells.
The cell theory states that all living things are made up of cells, that all cells come from previously existing cells, and that the cell is the simplest unit of life and is the foundation on which more complicated levels are built upon.
All living things are made up of cells, and cells are often classified as the simplest unit of life (the least organized level which still constitutes life). All of the other levels are built upon the cell, including tissues, organs, organisms, and even ecosystems.
Hope this helps!