Which component forms the outer layer of a cell?

The outer layer of a cell is primarily formed by the cell wall or cell membrane, making this choice the correct answer. In animal cells, the outermost boundary is the cell membrane, which is a phospholipid bilayer that regulates the movement of substances in and out of the cell and provides structural support. In plant cells, fungi, and some bacterial cells, the cell wall offers additional rigidity and protection beyond what the cell membrane provides.

The cell wall is typically composed of cellulose in plants, chitin in fungi, or peptidoglycan in bacteria, allowing it to fulfill various protective and supportive roles. On the other hand, the capsule is an additional layer that can be found in some bacterial cells, offering extra protection and aiding in adherence to surfaces, but it is not universally present and does not define the outer layer of all cell types.

The nucleus, which contains genetic material, is located inside the cell and is not part of the outer layer. Cytoplasm refers to the gel-like substance within the cell membrane that contains organelles but does not form an outer boundary. Thus, the cell wall or cell membrane is the defining structure of a cell's outer layer, making that the correct choice.