Answer:
B. A small lipid
Explanation:
Small lipids are nonpolar, so they can pass most easily through the nonpolar tails of the phospholipid bilayer.
A is wrong. Protein molecules are much too large to pass through the membrane.
C is wrong. Ions need special channels to pass through the membrane.
D is wrong. A water molecule is small enough to pass through the membrane. However, it is polar, so it can't do so as easily as a small lipid.
