Which of the following should pass most easily through the plasma membrane?
a. a protein
b. a small lipid
c. a sodium ion
d. a water molecule

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.

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