The cell theory, one of the fundamental principles in biology, was proposed independently by Matthias Schleiden and Theodor Schwann in the 1830s and 1840s.
Together, Schleiden and Schwann developed the cell theory, which states that:
. All living organisms are composed of one or more cells.
. The cell is the basic structural and functional unit of life.
. Cells arise from pre-existing cells through cell division.
This formulation of the cell theory by Schleiden and Schwann laid the foundation for modern cell biology and our understanding of the organization and functioning of living organisms.
Therefore, the correct answer is option (B) Schleiden and Schwann.