The part labelled I is the

The diagram shows an animal cell with four labelled parts. Label I points to the large, dark, roughly spherical structure located centrally within the cell. This is the nucleus, the membrane-bound organelle that contains the cell's genetic material (DNA) and controls cell activities such as growth, metabolism, and reproduction.

The other labelled structures can be identified as follows: Label II points to the cell membrane (the outer boundary), Label III indicates the cytoplasm (the gel-like substance filling the cell), and Label IV points to small, elongated organelles which are the mitochondria (responsible for cellular respiration and energy production).

The nucleus is distinguishable by its large size relative to other organelles, its dark staining (due to dense chromatin material), and its roughly circular shape. It is often the most prominent structure visible in a cell diagram.