The correct answer is Robert Brown.

Robert Brown, a Scottish botanist, is credited with the discovery of the cell nucleus. In the early 1800s, through meticulous observations using microscopes of plant cells, particularly orchids, he identified a consistently present, dense, and typically round structure within the cells. This structure, which he initially referred to simply as an "areola" before eventually settling on the term "nucleus", is now understood to be the control center of eukaryotic cells. Brown's detailed descriptions and illustrations significantly advanced our understanding of cell structure and laid a foundation for future cellular research, helping to establish the nucleus' crucial role in cellular processes and heredity. His discovery marked a pivotal moment in cell biology, distinguishing the nucleus as a distinct and vital component within the cellular architecture.

While Robert Hooke coined the term "cell" and Anton van Leeuwenhoek made significant contributions to microscopy and the observation of microorganisms, neither is directly credited with discovering the nucleus. Hooke's observations focused on the overall structure of cork, and Leeuwenhoek's work primarily revolved around single-celled organisms, neither specifically identifying the nucleus.