Cell theory was proposed by a German botanist, Matthias Schleiden and
English zoologist, Theodor Schwann in the year 1839. Cell theory was considered as most acceptable theory which, elucidated the functioning of cell. The cell theory includes the following points -
1) All living things are made up of cells.
2) The cell is the smallest living thing that can perform all the functions of life.
3) All cells must come from pre-existing cells.
So, the correct answer is option D.