Final answer:
The correct classifications for chiral amino acids are D or L for their 3D structure, and R or S for their biological standard format configuration. L-amino acids are used in biological systems to construct proteins, although they are often dextrorotary. Most naturally occurring chiral amino acids are S except for cysteine which is R, and glycine is not chiral. Therefore, correct option is a.
Step-by-step explanation:
Chiral amino acids can be classified as D or L when looking at them in a 3D structure, and as R or S when considering their configuration in a biological standard format. When viewed in the Fisher projection, amino acids with the -NH2 group on the right side are D-amino acids and those with it on the left side are L-amino acids.
Despite this naming, in biological systems, the L-amino acids are the ones that are predominantly used to build polypeptides and proteins and are often dextrorotary (i.e., they rotate plane-polarized light to the right), which would make them d-amino acids based on their optical activity.
In terms of stereochemistry, using the Cahn-Ingold-Prelog priority rules, most naturally occurring chiral amino acids have an S configuration, except for cysteine, which has an R configuration.
However, this R/S notation is strictly a descriptor of the three-dimensional arrangement of the atoms around the chiral center and does not necessarily correlate with optical rotation. The exception is glycine, which is not chiral and has no D or L, R, or S designation.