Correct Answer (Detailed Explanation is Below)
A.
Theodor Schwann
Detailed Explanation
Theodor Schwann, along with Matthias Schleiden, formulated major parts of the cell theory in the 1830s. Schleiden emphasized plants, while Schwann extended the idea to animals. Later,
Rudolf Virchow contributed the principle that new cells arise from pre-existing cells. Cell theory is a fundamental concept of modern biology.