Since -COOH group is attached to alanine, one terminal of the tetrapeptide contains alanine. Thus, its position is fixed.Glycine is achiral whereas valine and phenylalanine are chiral. Thus, -NH_2 group is attached to either valine or phenylalanine.Thus following are the four possible combinations of the tetrapeptide. H_2N-Val-Gly-Phe-Ala-COOH H_2N-Val-Phe-Gly-Ala-COOH H_2N-Phe-Gly-Val-Ala-COOH H_2N-Phe-Val-Gly-Ala-COOH Therefore, the correct option is A.