Final answer:
The aliphatic, hydrophobic, non-polar amino acid with a sulfur-containing side chain is methionine. It belongs to the group of nonpolar amino acids due to its thioether group in the side chain, which makes it hydrophobic despite the sulfur atom.
Step-by-step explanation:
The aliphatic, hydrophobic, non-polar amino acid with a side chain that includes a sulfur atom is methionine. Despite the presence of a sulfur atom in its side chain, methionine is considered non-polar and hydrophobic due to its thioether group. This characteristic allows it to be grouped among nonpolar amino acids. It is distinct from other sulfur-containing amino acids like cysteine, which also has a non-polar side chain but has the unique ability to form disulfide bonds through oxidation of its thiol group. This question does not involve tyrosine, which has a polar aromatic side chain that includes phenol, or leucine, which is purely aliphatic and hydrophobic without any sulfur.