Answer & Explanation
C) Robert Brown
Robert Brown, Scottish botanist, Discovered The Nucleus of a Cell in 1831, London. When Brown was studying orchids under microscope when he observed an opaque area, which he called the "Areola" or "Nucleus", in the cells of the flower's outer layer.
The nucleus was also described by Franz Bauer in 1804 but in detail Robert Brown made it in 1831.