Answer
d.
Work Step by Step
Protein and peptide hormones are those that are made of amino acids. Because these types of hormones tend to be polar, they can easily enter the blood, which is polar because of its high water content. However, these hormones have problems when encountering lipid bilayers, which prevent the passage of polar substances. Therefore, hormones made of amino acids must bind to receptors on the surface of their target cells.
