Which best describes the structure of a cell's plasma membrane?

There are two answers here that would work best: proteins sandwiched between two layers of phospholipids (A) and proteins embedded into a layer of phospholipids (B). This is because there are proteins inside the cell membrane that help stop things from getting in that aren’t supposed to but are small enough to get through unassisted, and because there are proteins embedded in the outer and inner layers to assist things that are too big to make it through the holes already provided.

The cell has been refined over thousands of years. Things have changed so that it works better now in a multi-cell organism than it could have a thousand, two thousand, even three thousand years ago. That said, it’s still very robust.