The naturally occurring proteins consist of

The correct option is L-amino acids.

Amino acids (with the exception of glycine) possess a central carbon atom, known as the α-carbon, which is bonded to four distinct groups:
1. An amino group (NH₂)
2. A carboxyl group (COOH)
3. A hydrogen atom (H)
4. A variable side chain (R-group)

Because the α-carbon is bonded to four different groups, it acts as a chiral center (or asymmetric carbon atom). This asymmetry allows amino acids to exist in two non-superimposable mirror-image forms called enantiomers. These two stereoisomers are designated as the L-configuration (left-handed) and the D-configuration (right-handed).

In all living organisms, the ribosomally controlled protein synthesis machinery (including tRNA and ribosome complexes) is highly stereospecific. It selects and incorporates only L-amino acids into polypeptide chains. Consequently, naturally occurring proteins are composed exclusively of L-amino acids. This homochirality is critical for the proper folding of proteins into stable secondary structures, such as α-helices and β-sheets, which are essential for their biological functions.

While D-amino acids do occur in nature (such as in the peptidoglycan cell walls of bacteria and in some peptide antibiotics), they are synthesized via specialized enzyme pathways rather than by the standard protein-building ribosomal translation machinery.