Which part of the cell is primarily involved in regulating what enters and exits the cell?

The cell membrane plays a crucial role in regulating the movement of substances into and out of the cell. Comprised primarily of a phospholipid bilayer with embedded proteins, it serves as a selective barrier. This means it allows certain molecules to pass through while preventing others from entering or leaving, thus maintaining homeostasis within the cell.

The cell membrane's selective permeability is essential for various cellular functions. For example, it regulates the intake of nutrients and the removal of waste products. Transport proteins within the membrane assist in moving specific ions and molecules across, either passively or actively, depending on the concentration gradient and energy requirements.

While the cytoplasm is the jelly-like substance inside the cell where many cellular processes occur, it does not have a regulatory function regarding the entry and exit of materials. The nucleus, which houses the cell's genetic material, is responsible for controlling cellular activities and gene expression but does not govern what crosses the cell boundary. Ribosomes are the sites of protein synthesis, crucial for producing proteins but not involved in the regulation of cellular transport.

Thus, the effectiveness and essential nature of the cell membrane in controlling the internal environment of the cell make it the correct choice for this question.