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 a 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.
Later
on, in year 1855 Rudolf Virchow added a new point in cell theory that
all cells must come from pre-existing cells, which was also known as
"Omnis cellula e cellula".
So, the correct answer is option D.