The correct answer is option 2: cell wall. The presence of a cell wall is what distinguishes a plant cell from an animal cell. A cell wall is a rigid, outer layer that provides support and protection to the plant cell. It is made up of cellulose and other polysaccharides and is found outside the cell membrane. The cell wall gives plant cells their characteristic shape and structure.

Option 1: Chloroplasts are present in plant cells but not in animal cells. Chloroplasts are responsible for the process of photosynthesis, where sunlight is converted into energy.

Option 3: The cell membrane is present in both plant and animal cells. It is a selectively permeable barrier that controls the movement of molecules in and out of the cell.

Option 4: The nucleus is present in both plant and animal cells. It is the control center of the cell, containing the DNA and information necessary for the cell`s functioning.

In summary, while options 1, 3, and 4 are present in both plant and animal cells, it is option 2, the cell wall, that is unique to plant cells.