The correct answer is: B. Robert Brown
Robert Brown was a Scottish botanist who first observed the nucleus in 1831. He was using a microscope to observe plant cells when he noticed a small, dark object in the center of each cell. He called this object the nucleus, and he hypothesized that it was the control center of the cell.
Robert Hooke was an English scientist who is considered to be the father of microscopy. He was the first person to observe cells, in 1665. However, he did not observe the nucleus.
Rudolf Virchow was a German physician who is considered to be the father of cell theory. He proposed that all cells come from preexisting cells, and he also observed the nucleus. However, he did not discover it.
Theodore Schwann was a German physiologist who is considered to be the father of cell theory. He proposed that all living things are made up of cells, and he also observed the nucleus. However, he did not discover it.
In conclusion, Robert Brown was the first person to observe the nucleus in 1831. He was a Scottish botanist who is considered to be one of the founders of cell theory.