The nucleus is labelled structure II. The nucleus is a membrane-bound organelle that contains genetic material (DNA) and regulates the activities of the cell by controlling gene expression. It is usually the largest organelle in eukaryotic cells and is responsible for the cell's growth, metabolism, and reproduction. In the given options, structure II is the only one that resembles the typical appearance of a nucleus with a round shape, distinct boundary, and a dense, central region called the nucleolus.
