Question

the phospholipid bilayer of membranes allows lipids and small, nonpolar molecules to pass across membranes, but prohibits the passage of ions and polar molecules. this occurs because the phospholipid tails are
always saturated.

hydrophilic and hydrophobic.

always unsaturated.

hydrophilic and selectively permeable.

hydrophobic and selectively permeable.

Explanation:

Phospholipid tails are nonpolar, meaning they are hydrophobic (water-fearing). This hydrophobic core of the bilayer repels polar molecules and ions, while allowing nonpolar/lipid-soluble substances to pass, creating selective permeability. The incorrect options are wrong because: tails are not always saturated/unsaturated, they are not hydrophilic, and selective permeability describes the membrane's property rooted in the tails' hydrophobic nature.

Answer:

hydrophobic and selectively permeable.