The cell theory states that

Explanation-

Cell theory was put forward by the scientist Schleiden and Schwann (1839)

Fundamental features of Cell Theory-
All living organisms are composed of cells and the product of cells.
Cells are structural and functional units of all living organisms.

Additional Information-

All cells are made up of a mass of protoplasm-containing nuclei and have a plasma membrane with or without a cell wall around them.

Final Answer-

So, The correct option is D.