Which of the following amino acids is nonpolar (hydrophobic)?

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Polarity of the side chain determines whether an amino acid behaves as hydrophobic or hydrophilic in water. Glycine has the simplest side chain—just a hydrogen atom—so it lacks polar groups and is nonpolar, tending to be hydrophobic. The other three have polar or charged groups: serine has a hydroxyl that makes it polar; lysine carries a positively charged amine; aspartic acid has a negatively charged carboxylate. These features make them hydrophilic. So glycine is the nonpolar, hydrophobic choice.