Which is a function of the cell membrane? a. performs photosynthesis b. controls what enters and leaves the cell c. protects the external structures of the cell d. provides energy to the cell

The right answer is- B. controls what enters and leaves the cell.

Cell membrane also termed as plasma membrane is a membrane of two layers of lipids and protein that surrounds all cells and separates the internal compartment of the cell from its external environment.

Cell membrane is a semipermeable membrane as it controls the movement of substances into and outside of the cell. In other words, it does not permit the substances that are useful to leave the cell and  substances that are harmful to enter the cell.