Question

what is the main function of the cell membrane in a plant cell?
to make proteins
to give strength and stiffness to the cell
to give the cell its fixed shape
to control which substances enter and leave the cell

Explanation:

The cell - membrane is semi - permeable, regulating transport. Making proteins is done by ribosomes, giving strength/stiffness and fixed shape is mainly due to the cell wall in plants.

Answer:

to control which substances enter and leave the cell