Final answer:
Alanine is the most water-soluble amino acid among the ones listed because it has a polar uncharged side chain which makes it hydrophilic, contrasting with the hydrophobic nature of the other listed amino acids.
Step-by-step explanation:
The student's question is about determining which of the given amino acids would be most soluble in water. When comparing the solubility of amino acids in water, we generally find that amino acids with polar or charged side chains are more soluble than those with nonpolar side chains. In the provided list, alanine (1), leucine (2), phenylalanine (3), and proline (4), alanine is the most water-soluble amino acid because it has a polar uncharged side chain, making it more hydrophilic compared to the others which are more hydrophobic
The hydrophobic amino acids such as leucine, phenylalanine, and proline are likely to be found in the interior of proteins, whereas hydrophilic amino acids like alanine would be found on the surface of soluble proteins and are more water-soluble.