Final answer:
The amino acid with a beta-branched side chain is valine (A). Valine has a nonpolar, hydrophobic side chain, distinct from the other options (serine, glycine, and phenylalanine) cited.
Step-by-step explanation:
The amino acid that exhibits a beta-branched side chain is valine (A). Amino acids like valine have aliphatic side chains, meaning they contain chains of carbon atoms. Valine's side chain is nonpolar and hydrophobic, which helps determine its role and function within proteins. It is part of a group of amino acids that commonly appear in the beta sheets of secondary protein structures. Serine, glycine, and phenylalanine do not have beta-branched side chains. Serine has a polar side chain featuring a hydroxyl group. Glycine's side chain is a single hydrogen atom, making it the simplest amino acid. Phenylalanine has an aromatic side chain, which is larger and not beta branched.