What are some location of amino acid

 They can be classified according to the core structural functional groups' locations as alpha- (α-), beta- (β-), gamma- (γ-) or delta-(δ-) amino acids; other categories relate to polarity, pH level, and side-chain group type (aliphatic, acyclic, aromatic, containing hydroxyl orsulfur, etc.). In the form of proteins, amino acids comprise the second-largest component (water is the largest) of human muscles, cells and other tissues.[5] Outside proteins, amino acids perform critical roles in processes such as neurotransmitter transport and biosynthesis.

Ithink the answer is d.