Which part of the cell membrane is responsible for preventing unwanted materials from entering the cell

LIPID BILAYER
 
The most important property of lipid bilayer is that it is highly impermeable structure. Impermeable means that it do not allow molecules to pass freely across the cell membrane. Water and gases can easily pass through the lipid bilayer. With this property means that large molecules and polar molecules cannot cross the bilayer. This property can be due to the structure of the lipid bilayer its asymmetrical, they also have different lipid and protein composition.

Icant be totally sure but i would say with confidence that it is example #1 about cells.