The correct answer is a. Cell Wall.
Plant cells have a rigid cell wall made of cellulose, which is not membrane-bound. In contrast, animal cells do not have a cell wall; they only have a flexible plasma membrane.
Chloroplasts, on the other hand, are membrane-bound structures found in plant cells and some protists but are absent in animal cells. Therefore, the structure lacking a membrane that is present in plant cells and not in animal cells is the cell wall.