The plasma membrane allows some molecular species to enter and excludes others. it is therefore referred to as a(n) membrane.

Semi-permeable

Explanation:

The cell membrane are the semi-permeable membranes which are made up of proteins and phospholipids. The semi-permeable membrane causes some molecules or ions to pass through the membrane and protects the other to enter inside. This process avoids harmful ions and molecules to pass enter inside the cell and cause damage to the cell.  This process helps in maintaining the cell metabolism.    

instead of copying and pasting things off of tests or work done on computer which im sure you did, try and read over the question carefully