The right option is; d. Schleiden and Schwann
(Theodor) Schwann and (Matthias) Schleiden first recognized the cell as the universal unit of life. In 1839, they developed the cell theory which identifies the cell as the most basic unit of plants and animals. Schwann and Schleiden were able to discover that some organisms are unicellular, while some are multicellular. They also discovered that the nucleus and the cell membrane are part of the features of the cell.