Final answer:
Matthias Schleiden discovered that all plants are made of cells, contributing to the development of the cell theory, which states that all living things are composed of one or more cells and new cells arise from existing cells. Hence the correct answer is option D
Step-by-step explanation:
The discovery that all plants are made of cells and contributed to the development of the cell theory was made by Matthias Schleiden. He proposed that cells are the basic unit of structure and function of all life, along with zoologist Theodor Schwann. Later, Rudolf Virchow observed that cells divide to produce more cells. Collectively, these observations form the Cell Theory, which states that all living things are made up of one or more cells and new cells arise from existing cells.
Hence the correct answer is option D