Which of the following is an essential amino acid?

• A. Alanine
• B. Aspartic acid
• C. Glutamic acid
• D. Valine

Answer: (D)

Essential amino acids are those we cannot make in sufficient amounts and must obtain from the diet. Among the options, alanine, aspartic acid, and glutamic acid can be synthesized by the body, so they are nonessential. Valine cannot be produced in adequate amounts and must be supplied by food, making it an essential amino acid. Valine is one of the branched-chain amino acids important for muscle protein synthesis and energy, which is why it fits as the essential amino acid in this group.