Which of the following is an amino acid:

Correct option is D

Explanation of each option:
· (a) Cysteine
· A sulfur-containing amino acid.
· Plays a key role in protein structure through disulfide bonds.
· (b) Histidine
· An essential amino acid (especially important in growth).
· Contains an imidazole ring and is important in enzyme active sites.
· (c) Proline
· A unique amino acid with a cyclic (imino) structure.
· Causes bends in protein chains and influences protein folding.
· Glycine
· The simplest amino acid, having only a hydrogen atom as its side chain.
· Provides flexibility to protein structures.
�� Key Points to Remember
· Amino acids are the building blocks of proteins.
· All amino acids contain:
· An amino group (–NH₂)
· A carboxyl group (–COOH)
· A central carbon atom
· Cysteine, histidine, proline, and glycine are all protein-forming amino acids.