Answer: The correct answer for the blank is -
the absence of a cell wall in cheek cells.
Cheek cells are the animal cells whereas onion cells are the plant cells.
Cell wall can be described as a rigid layer, composed of polysaccharides (such as cellulose in higher plants) and present outside the plamsa membrane ( also called cell membrane) in case of plants, fungi, and bacteria.
It is absent in animal cells, such as human cheek cells (which possess only cell membrane, surrounding the cytoplasm of cell).
Thus, cell wall is absent in cheek cells whereas present in onion cells.