The correct answer is option 1, the cell membrane. The cell membrane is a thin, flexible barrier that surrounds the cell and separates the internal environment of the cell from the external environment. It is also known as the plasma membrane.

The cell membrane is semi-permeable, which means that it allows certain substances to pass through while restricting the passage of others. This selective permeability is essential for maintaining the balance and integrity of the cell. Small molecules like oxygen and carbon dioxide can pass through the cell membrane freely, while larger molecules and ions require specific transport mechanisms to enter or exit the cell.

Option 2 is the same as option 1, as the plasma membrane and cell membrane refer to the same structure.

Option 3, the cell wall, is not semi-permeable. The cell wall is a rigid structure found in plant cells and some other organisms. It provides support and protection to the cell but does not regulate the passage of substances like the cell membrane does.

Option 4, the nucleus, is the control center of the cell and contains the cell`s genetic material. However, it is not semi-permeable as it is surrounded by a double membrane called the nuclear envelope, which has pores that allow the passage of molecules in and out of the