Final answer:
The Cellular Theory is the correct answer, summarizing the work of Schleiden, Schwann, and Virchow by stating that all living things are made up of cells and that new cells originate from existing cells.
Step-by-step explanation:
The Cell Theory combines the conclusions made by Matthias Schleiden, Theodor Schwann, and Rudolf Virchow. These three scientists contributed to our current understanding that all living things are composed of one or more cells, the cell is the basic unit of life, and new cells arise from existing cells. Therefore, the correct answer to the question is (a) Cellular Theory.