The correct answer is B. 1839.
The German physiologist Theodor Schwann and the German botanist Matthias Jakob Schleiden independently proposed in 1839 that all living things are composed of cells. This is known as the cell theory.
Schwann’s work was based on his studies of animal tissues, while Schleiden’s work was based on his studies of plant tissues. They both observed that all living things are made up of cells, and that cells are the basic units of life.
The cell theory is one of the most important concepts in biology. It has helped us to understand how living things work, and it has led to many important advances in medicine and other fields.
Option A is incorrect because it is the year that Schleiden first proposed that cells were the basic unit of life. However, Schwann did not make this proposal until 1839.
Option C is incorrect because it is the year that Schwann and Schleiden published their findings on the cell theory. However, they did not first propose that cells were the basic unit of life until 1839.
Option D is incorrect because it is the year that Virchow proposed that all cells come from preexisting cells. This is known as the cell doctrine, and it is a further development of the cell theory.