Hiya,
The plasma membrane is sorta like the military of the cell. It's function (it's meaning) is to serve and protect the cell from it's scary surroundings! Composed of proteins, it is very selectively permeable. Meaning it allows certain types of molecules or even ions to pass on through it, as in, active or passive transport.
So, the answer might be...
A.) It is selectively permeable to certain substances.
Hope this helps!