The correct answer is option 3, cell wall.

Option 1, cell membrane, is incorrect because both animal and plant cells have a cell membrane. The cell membrane is a protective barrier that controls the movement of substances in and out of the cell.

Option 2, endoplasmic reticulum, is incorrect because animal cells do have endoplasmic reticulum. The endoplasmic reticulum is responsible for protein synthesis and lipid metabolism in cells.

Option 4, mitochondria, is incorrect because animal cells do have mitochondria. Mitochondria are known as the powerhouses of the cell, producing energy in the form of ATP.

Therefore, option 3, cell wall, is the correct answer. The cell wall is a rigid layer that provides structural support and protection for plant cells. Animal cells do not have a cell wall.