Final answer:
Theodor Schwann extended Schleiden's hypothesis about plant tissue by suggesting that animal tissue was also made up of cells, contributing significantly to the establishment of cell theory. The correct answer is B. Theodor Schwann.
Step-by-step explanation:
The person who extended Matthias Schleiden's hypothesis by suggesting that animal tissue was made up of cells was Theodor Schwann.
Schleiden had already proposed that plant tissue consisted of cells, and after a conversation with Schleiden, Schwann suggested that this concept also applied to animal tissues.
Cell theory, proposed by Schwann and Schleiden, marked a significant advancement in biological sciences, allowing us to understand that both plant and animal tissues are composed of cells, leading to the realization that cells are the basic unit of life in all living organisms.
Furthermore, the contributions of Rudolf Virchow later reinforced this foundation by asserting that all new cells arise from existing cells.