Final answer:
Glycine is the only one of the 20 standard amino acids which is optically inactive because its α-carbon does not have four different substituents and thus lacks a chiral center.
Step-by-step explanation:
The amino acid which is optically inactive is glycine. This is because, unlike the other 19 standard amino acids, glycine's α-carbon is not bound to four different groups. Therefore, it does not have a chiral center and is not optically active. In the other amino acids, the α-carbon is bound to an amino group, a carboxyl group, a hydrogen atom, and a variable R group, making them chiral and capable of existing in either the D- or the L-enantiomeric form. However, glycine has two hydrogen atoms attached to its α-carbon, so it lacks the necessary four different groups for chirality and does not rotate plane-polarized light.