The cell membrane. (It also could be cytoplasm because it supports the organelles. But I think it is the cell membrane because it says barrier, and the cell membrane is the cell's barrier between it and the outside of the cell.)
The cell membrane is the outer most layer of the animal cell. It is selectively permeable, so it lets good things in and keeps bad things out. It also helps larger things that the cell needs to get in such as glucose by the process of endocytosis.
