The cell membrane is called selectively permeable. Why?

Hint: Permeability is the ability of any membrane to allow substances (mostly solutes and solvents) to pass through it. Depending upon the permeability, the membranes are classified into: freely permeable, semi-permeable, selectively permeable and impermeable membranes.

Complete answer:
To know the correct answer to the question, we must be well aware of the different types of membrane with respect to their permeability.
We know that the membrane is the outermost covering of the cells, organelles etc. The membrane provides a passage for the entry and exit of the various metabolites and other substances in and out of these cells and the cell organelles.
There are certain substances which are required by the cell while there are some which have no use in the cell. To filter between such particles or substances, the membrane has a special feature. It allows the entry of only those substances which are required by the cell while holding back the others. Same happens on the inner side of the cell. The excretory matter and other substances which are not required by the cell are thrown out of the cell through this membrane while those which the cell cannot afford to lose are held back.

Therefore, the cell membrane is semi-permeable.

Note: Here the semi-permeable membranes should not be confused with the selectively permeable membranes. Both are two different things. While semi-permeable membranes allow some solutes and almost all solvents to through them, selectively-permeable membranes might even hold back the solvents and some solute particles as well.