The correct answer is option 4, cell wall.

Cell walls are a characteristic feature of plant cells and are not found in animal cells. A cell wall is a rigid layer that surrounds the cell membrane of plant cells, providing structural support and protection. It is made up of cellulose, hemicellulose, and pectin.

Option 1, free ribosomes, are found in both animal and plant cells. They are responsible for protein synthesis and can be found freely floating in the cytoplasm.

Option 2, mitochondria, are present in both animal and plant cells. They are the powerhouse of the cell, producing energy in the form of ATP through cellular respiration.

Option 3, nucleolus, is a structure found within the nucleus of both animal and plant cells. It is responsible for the production and assembly of ribosomes.

In conclusion, the cell wall is not found in animal cells, while free ribosomes, mitochondria, and nucleolus are present in both animal and plant cells.