Question

1. the nature of the atom or molecule that encounters the membrane; and
2. the structure of the membrane itself.

lets focus on the second issue for now.
it should make sense to you that lipid bilayers that have stronger hydrophobic interactions are less permeable than bilayers with weaker hydrophobic interactions. and you should recall that the strength of the hydrophobic interactions in the interior of a membrane is a function of hydrocarbon tail length and saturation and the amount of cholesterol present.
(as an aside, you might notice that the readings will start to reference \lipid bilayers\ instead of \phospholipid bilayers,\ to recognize that cholesterol—or other steroids, depending on the organism in question—occur along with phospholipids in cell membranes.)
which of these lipid bilayers is the most permeable?

Explanation:

Lipid bilayers with weaker hydrophobic interactions are more permeable. Hydrophobic interactions are stronger with longer - hydrocarbon tails, more saturated tails, and more cholesterol. Option C has no cholesterol and more unsaturated phospholipids, which lead to weaker hydrophobic interactions and higher permeability.

Answer:

C.