I believe the answer is A. hydrophobic. Inside the biological membranes are lipids (hydrophobic) molecules. As you may know the membranes are lipid bilayers. In order to place proteins, which are polymers of amino acids, you would need to create an like-to-like environment. The R groups of amino acids will interact with this hydrophobic lipid bilayer environment and will need to display the same characteristics to keep the plasma membrane intact and not cause unwanted disruption. Therefore, R groups of amino acids in biological membranes should be hydrophobic.