The most important function of a cell membrane is that it

Question

$\begin{array}{1 1} (a)\; \text{Controls the entry and exit of materials} \\ (b)\; \text{Controls only the entry of materials into cells} \\ (c)\; \text{Controls only the exit of materials from the cell} \\ (d)\; \text{Allows entry and exit of materials without control} \end{array}$