The plasma membrane is composed of phospholipid

I believe the answer is B,( Allowing specific substances to flow into the cell).

Membrane protein is found in all types of cell membrane, it can either be integral protein, which is embedded within the membrane or associated protein, which is associated with the surface of cell membrane of phospholipid bilayer.

It performs several functions such as cell to cell communication, molecular transport and cell signaling.

Embedded proteins are proteins which are found in the cell membrane, which are completely embedded in the bilayer membrane of the cell membrane. It consists of hydrophilic head and hydrophobic tail. The head face towards the water molecule and tail face inward away from the water molecule. These protein help in the movement of molecules in and out of the membrane.

Example of embedded proteins: Channel proteins and carrier proteins and are example of some of the embedded proteins.

I apologize if this is not what you meant.