Question

the cell membrane forms a protective barrier between the external and internal environments. it is referred to as ____________ because it allows some substances to cross more easily than others and blocks other substances from crossing into the cell.
a impermeable
b a fluid mosaic
c a plasma membrane
d selectively permeable

Explanation:

The cell - membrane's property of allowing some substances to pass through more easily than others and blocking some is called selective permeability. Impermeable means nothing can pass through. A fluid mosaic refers to the structure of the cell - membrane. Plasma membrane is another name for the cell - membrane but doesn't describe the property in question.

Answer:

D. selectively permeable