What substance acts as a protective outer layer in certain types of cells?

The correct answer is the cell wall, which serves as a protective outer layer in certain types of cells, particularly in plants, fungi, and bacteria. The cell wall provides structural support and rigidity, helping to maintain the shape of the cell and protect it from mechanical stress and osmotic pressure. It is made primarily of polysaccharides; for instance, plant cell walls are mainly composed of cellulose, while bacterial cell walls contain peptidoglycan.

In contrast, the cell membrane, often referred to as the plasma membrane, is a thin, flexible layer that surrounds all cells, serving as a barrier to separate the interior of the cell from its external environment. While it does provide some protection and plays a vital role in regulating what enters and exits the cell, it is not as rigid and protective as the cell wall.

The term "integument" typically refers to the outer covering of an organism, which can include skin, shells, or other protective layers but does not specifically denote the protective structure present in certain types of cells.

By understanding the function of the cell wall, it becomes clear why it is identified as the protective outer layer for the specific organisms that possess it, in contrast to the other options that have different roles or contexts.