The correct answer is option 2: Cell wall.

The presence of a cell wall is what primarily differentiates plant cells from animal cells. Plant cells have a rigid cell wall composed mainly of cellulose, which provides support and protection to the cell. The cell wall is located outside the cell membrane. In contrast, animal cells lack a cell wall and instead have a flexible cell membrane that surrounds and protects the cell.

Option 1: Ribosomes are present in both animal and plant cells. Ribosomes are responsible for protein synthesis.

Option 3: Chloroplasts are only found in plant cells. They contain chlorophyll, which is necessary for photosynthesis, the process by which plants convert sunlight into energy.

Option 4: Protoplasm is a general term that refers to the living contents within a cell, including the cytoplasm and nucleus. Both animal and plant cells contain protoplasm.

In summary, the difference between animal and plant cells lies in the presence of a cell wall, which is exclusive to plant cells.