The correct answer is nucleus. The structure "C" refers to the nucleus, which is a membrane-bound organelle found in eukaryotic cells. It contains the cell's genetic material, DNA, and is responsible for controlling the cell's activities. The nucleus plays a vital role in cell division, gene expression, and the overall regulation of cellular functions. Chromosomes are found within the nucleus, but they are not the structure itself. Chloroplasts, on the other hand, are organelles found in plant cells that are responsible for photosynthesis and are not related to the structure "C".